Review: Britax Römer KIDFIX III M Highback Booster Car Seat

There are many milestones that make us parents realise that our kids are growing up. One of the recent ones for me has been my daughter’s switch from a baby / toddler car seat (Group 0+/1) to a  highback booster (Group 2/3). As she’s recently turned 4-years old, it was time to palm off her trusty Britax DUALFIX and DUALFIX i-SIZE car seats to her brother and introduce her to a new one – the Britax KIDFIX III M.

It may not sound like much of a milestone, but it is for us because it’s the first time she’s been forward facing. We’re huge proponents of extended rear facing due to the proven safety benefits, so L has spent her life to date looking out of the back window.

As such, choosing the right forward facing car seat was vital for us. We wanted something that used ISOFIX, something that would do her until she was a lot older and something that provided safety and security. Luckily, Britax had just the thing and sent it to us to review as part of their Britax Blogger Ambassador Programme.

The Britax KIDFIX III M is from their Premium Line, is based on the popular KIDFIX II XP SICT and is exclusive to Mothercare. Below, you’ll find my review of the KIDFIX III M, which we’ve been using for a couple of months now – mainly in a Volkswagen Tiguan, but also in a Kia Stonic.

Product Description

The product is described as:

The stylish and comfortable Kidfix III M extends Britax’ popular and successful Kidfix range. With its new design and longer seating area it offers more comfort for small as well as taller children – even on longer journeys. It also combines the proven safety features SICT and SecureGuard for excellent protection, ensuring children from 3.5 to 12 years are safe and comfortable on every journey.

Product Experience

The Britax KIDFIX III M came already assembled in the box. Unlike car seats for younger kids where you might have things like a separate ISOFIX base or a newborn insert, this one was ready to go from the start which certainly made a pleasant and unexpected change!

First impressions were good. We’d chosen the car seat in Midnight Blue (other colours are available), which I felt gave it a stylish and modern design which worked well with the black shell and red accents (more on those later). It also felt padded, comfortable and well built, plus looked to provide good protection around the sides and head.

This is a Group 2/3 car seat, meaning that it’s suitable from 15 to 36 kg, which is roughly around 3.5 to 12 years old. That felt like a massive difference in age and size, but I soon saw how the KIDFIX III M can grow with your child as I adjusted the headrest to minimum and maximum heights (below).

Review Britax Römer KIDFIX III M Highback Booster Car Seat side and front headrest up and down

The Britax KIDFIX III M can be installed in one of two ways – (1) with ISOFIT and the 3 point belt, or (2) with just the 3 point belt. Both methods utilise the seat belt of the car, however the former also uses the ISOFIX fastening points in the car to secure the car seat to the vehicle. This is what we’ve always chosen to do, and have found it simple and quick to achieve.

To install, put the car seat into the back of the car facing forwards. First, you then need to secure the car seat to the car – find and squeeze the grey lever at the front of the seat cushion (the ISOFIT adjustment button), which allows you to extend the ISOFIT locking arms out at the back of the seat.

These days, a lot of cars come with ISOFIX insertion aids as standard, however there’s two in the box if they’re required. These aids basically allow you to see the ISOFIX fastening points on the car, protect the seat from damage and make it easier to connect the ISOFIT locking arms to the ISOFIX fastening points.

When ready, ensure the back rest of the car seat is flush with the back seat of the car – depending on your car, you might need to remove the headrest at this point so that the KIDFIX III M can lie flush.

Once flush, line up the ISOFIT locking arms with the ISOFIX fastening points, then push the car seat backwards so that the locking arms engage with the fastening points. You’ll hear an audible click and notice that there’s now a green indicator on both ISOFIT locking arms to show that it’s fitted properly.

Review Britax Römer KIDFIX III M Highback Booster Car Seat ISOFIT Locking arms attached to ISOFIX fastening points

Finally, hold the ISOFIT adjustment button at the front of the car seat and push the base of the car seat backwards as far as it’ll go. Give it a wiggle to ensure it’s secure, check the two green indicators on the ISOFIT locking arms, then you’re done with car seat installation. It’s literally a 15 second job.

The next step is to safely secure your child into the KIDFIX III M using the car seat belt. To aid this step, the highback booster is cleverly coloured with bright red and dark red sections which give an indication of where the seat belt should go. Very handy.

First, you’ll want to take the seat belt, pull it outwards around your kid and clip it into the car’s seat belt buckle. Make sure it’s not twisted, then guide both the lap and diagonal parts of the seat belt into the bright red belt guide nearest to the buckle (by L’s hand in the photo below).

Review Britax Römer KIDFIX III M Highback Booster Car Seat plugged into buckle

Next, put the lap belt part of the seat belt into the bright red SecureGuard. This is a little, upside down hook between your child’s legs which gives the seat belt four contact points rather than the usual three. As such, this helps ensure the lap belt is in the optimal position and protects the abdominal area. Once done, ensure the lap belt is also in the bright red belt guide on the opposite side.

Review Britax Römer KIDFIX III M Highback Booster Car Seat secureguard

The final step is to feed the diagonal part of the seat belt into the dark red belt holder. You’ll find this behind the headrest on the side where the seat belt is at the highest point. Depending on the height of your child and headrest, you might need to move the headrest up temporarily to find it (using the adjustment handle on the back of the headrest) before moving back down again.

Review Britax Römer KIDFIX III M Highback Booster Car Seat seat belt in belt holder

Once the seat belt is correctly on / in the bright red and dark red sections of the KIDFIX III M, you can tighten the belt around your child by pulling on the diagonal section of the belt. You’ll then want to unscrew the SICT on the side closest to the door – this is the big, plastic circle which is designed to provide protection in case of any side impacts and is something I first discovered on the Britax BABY-SAFE i-SIZE.

Review Britax Römer KIDFIX III M Highback Booster Car Seat SICT unscrewed

It may sound like there’s a number of steps to securing your child, but it takes little more than a few seconds and is very simple once you’ve done it once or twice. In addition, if you’re keeping the KIDFIX III M in the car permanently, you’ve got no reason to screw the SICT in and out each time or to remove the diagonal part of the seat belt from the dark red belt holder, which saves time and effort.

Review Britax Römer KIDFIX III M Highback Booster Car Seat child securely in place

All in all, I’m really impressed with the car seat. One of the things I like is the focus on comfort, particularly as we do a fair few long journeys. For instance, there’s a longer seating area (Britax claim 20% larger than similar models) with a soft front which provides comfort and an an ergonomic leg position. In addition, it has a wider v-shaped backrest which gives a cosy fit for young kids but then more space as they grow, as well as the ergonomic head rest.

Luckily, we’ve not been in an accident so I (obviously!) haven’t tested the safety of the car seat. However, I really trust Britax and know safety is paramount for them as they’re always pushing boundaries. This is shown with the SICT for side impacts, the SecureGuard to protect the abdomen, the large side wings and black shell for additional protection.

L is also really happy in it and is loving the fact that she is now forward facing. Safety still remains key and we plan for her brother to be extended rear facing until 4-years old as well, but there’s no doubt that forward facing has opened up a whole new world for her when we’re in the car.

However, I do have a few niggles based on personal preference. Firstly, although I know it’s totally safe using the car seat belt, it feels weird not having a 5-point harness. She’s had one all of her life and the car seat we use in our second car uses one – although that’s up to the age of seven (Group 0/0+, 1, 2) and not 12.

Because she’s just in a seat belt, it’s easier for her to move around (e.g. to reach over to her brother) or to unbuckle herself from the car seat. She’s a sensible kid and knows not to do this until we’ve stopped and have give her permission, but it provides a safety concern as young kids can do erratic things.

Secondly, due to the slope of the back seat in our car and the fact that the KIDFIX III M uses an articulated axis to connect the base to the backrest, we’ve found movement in the backrest, e.g. if the brakes are suddenly applied, L’s body will be pushed forward an inch or so as the momentum of the car pushes the backrest forward.

I’ve had it confirmed by Britax that it’s a safe install in our car, but it must be a bit annoying for L – particularly when she’s asleep. Talking about sleep, my third niggle is that her head can fall forward when she’s asleep and she ends up looking quite slumped. She has no issue falling asleep so it can’t be that bad, but I’d prefer to see her head on the headrest more often when she’s in the land of nod.

Review Britax Römer KIDFIX III M Highback Booster Car Seat child asleep


The Britax KIDFIX III M is exclusive to Mothercare and would set you back £180. I was pretty surprised at this as I thought it would be more expensive considering that it covers such a large age range (3.5 to 12 years old). Like most things kid-related, you can get cheaper and more expensive alternatives – the Britax KIDFIX III M is on the more premium side of things, however, for that, you get multiple safety features not included in cheaper models. Ultimately, with something like this, you get what you pay for and I’d be happy to pay more for a superior car seat.


I was a big fan of Britax before testing this car seat – and I continue to be after owning it for a few months. Going forward facing after four years of rear facing was a big move, but I have total confidence in Britax and their car seats.

I didn’t have much experience of highback boosters prior to this, but the new KIDFIX III M has certainly shown me what a good one should do. Firstly, installation is easy and I love that this utilises ISOFIX. Secondly, it has multiple additional safety features such as the SICT and SecureGuard, which gives me confidence if we ever were in an accident. It also grows with your child via an extending headrest, a wide backrest and larger seating area. Then, finally, it is great to look at and very comfortable to sit in – so L tells me.

As mentioned, I have one or two minor gripes, such as it using the seat belt rather than an inbuilt harness and the fact that the backrest has a bit of movement. However, I realise these are mainly personal preference and they barely detract from what is a brilliant car seat. If you’re in the market for a highback booster, then you’re not going to go wrong with the KIDFIX III M.

The DADventurer Star Rating

4.5 out of 5 Stars


Review: Disclosure: The KIDFIX III M was sent to me by Britax with the purpose of writing an honest review in exchange for the product as part of the Britax Blogger Ambassador Programme.