If your child has outgrown their harnessed car seat, a high-back booster is the next step, and two names come up again and again: the Britax Römer Kidfix i-Size and the Cybex Solution G2. Both are i-Size (R129) high-back boosters for children of roughly 3 to 12 years, both use ISOFIX connectors, and both are built around side impact protection. The short answer: the Kidfix i-Size is the pick if you want the most safety technology for one family car, while the Solution G2 suits families who move the seat between cars, taxis and holidays because it folds compactly and is easy to carry.
Below we compare them side by side, explain what the UK rules say, and suggest two alternatives if neither is quite right.
What UK law says about booster seats
Before comparing models, it helps to know the rules. According to GOV.UK, children must normally use a child car seat until they are 12 years old or 135 centimetres tall, whichever comes first. Only approved height-based seats can be used in the UK, and they carry a label with a capital E in a circle and "R129".
GOV.UK also notes that manufacturers can now only make booster cushions (the backless type) approved as group 3, for children from 22kg. A high-back booster gives your child head and side support that a cushion cannot, which is why most parents move from a harnessed seat to a high-back booster rather than straight to a cushion.
In a high-back booster, your child is held in place by the car's own three-point adult seat belt. The booster raises your child so the belt sits in the right position and guides it across the body. On seats like the Kidfix i-Size and Solution G2, the ISOFIX connectors attach the booster to the car to keep it stable, and it stays secured even when your child is not sitting in it. Always follow the manufacturer's instructions for routing the belt.
Kidfix i-Size vs Solution G2 at a glance
| Britax Römer Kidfix i-Size | Cybex Solution G2 | |
|---|---|---|
| Regulation | i-Size (R129) | i-Size (R129) |
| Child size | Approx. 3.5 to 12 years (100–150 cm) | Approx. 3 to 12 years (100–150 cm) |
| Fitting | ISOFIX connectors + adult 3-point belt | ISOFIX connectors + adult 3-point belt |
| Headline safety features | XP-PAD, SecureGuard, Side Impact Cushion Technology | Side impact protection, adjustable headrest |
| Folds for travel | No | Yes, compactly, with an integrated carry handle |
| Best for | One main family car, maximum safety tech | Switching cars, taxis and holidays |
Britax Römer Kidfix i-Size: safety technology first
Britax describes the Kidfix i-Size as its safest Kidfix model to date, developed with car manufacturers for children from around 3.5 to 12 years. Its key features are:
- XP-PAD: a pad on the seat belt that, according to Britax, reduces impact forces to the neck and chest by up to 30%.
- SecureGuard: an extra belt guide that helps keep the lap part of the belt in the correct position, low across the hips.
- Side Impact Cushion Technology (SICT): absorbs energy from a side impact before it reaches your child.
- Adjustable headrest with height indicators, so you can see when it is set correctly as your child grows.
- Ventilated seat shell and soft padding for comfort on longer journeys.
Britax also states that the Kidfix i-Size has achieved top scores in ADAC safety tests. Its slim design helps it fit a wide range of cars. The trade-off is that it is a fixed-shell seat: it is not designed to fold, so it is best left installed in your main car.
Cybex Solution G2: built for families on the move
The Cybex Solution G2 i-Size is the next generation of the popular Solution G i-Fix, for children from around 3 to 12 years (100–150 cm). Its standout feature is portability:
- Folds compactly in seconds, so it fits in a boot, wardrobe or holiday luggage space.
- Lightweight and self-standing, with an integrated handle for carrying.
- ISOFIX connectors for a stable fit in cars with ISOFIX points, with the child secured by the adult seat belt.
- Adjustable headrest that grows with your child.
If your child regularly travels with grandparents, in a second car or in taxis, being able to fold the seat and carry it with one hand makes a real difference. There is also a Solution G2 Plus version; check the product page for how it differs from the standard model.
Which should you choose?
Choose the Kidfix i-Size if…
- The seat will stay in one car most of the time.
- You want belt-positioning and impact-absorbing features such as XP-PAD and SecureGuard.
- You want a slim seat designed with car manufacturers.
Choose the Solution G2 if…
- You move the seat between cars, use taxis or travel abroad.
- Storage space at home or in the boot is tight.
- You want a lightweight seat your older child can help carry.
Whichever you choose, check your car is compatible: ISOFIX positions and seat belt buckle lengths vary between models, and manufacturers publish vehicle fitting lists.
Two alternatives worth considering
The Maxi-Cosi Kore i-Size is a high-back booster for 3.5 to 12 years (100–150 cm) with ISOFIX, adjustable height and width, and Side Protection System Plus. Its open seat design and intuitive belt guide make it easy for older children to buckle up on their own.
The Silver Cross Discover i-Size covers 3 to 12 years (100–150 cm), with ISOFIX installation, side impact protection and an adjustable headrest and backrest that grow with your child.
You can compare all of our high-back boosters and other baby and toddler car seats in one place. For more background, read our guides to moving your child up to a booster seat and ISOFIX booster seats for growing kids.
FAQ
When should my child move to a high-back booster?
Move up when your child outgrows the height or weight limit of their harnessed seat, as stated in that seat's manual, and fits the booster's range. i-Size high-back boosters like these start from 100 cm. There is no need to move up early.
Is ISOFIX safer on a booster seat?
On a high-back booster, the adult seat belt holds your child. ISOFIX connectors attach the booster to the car so it stays stable and does not move around when it is empty. Many parents find this reassuring, but the belt must still be routed correctly every journey.
How long does a child need a car seat in the UK?
According to GOV.UK, until they are 12 years old or 135 cm tall, whichever comes first.
Can I use a booster seat in a taxi?
A foldable, lightweight booster such as the Cybex Solution G2 is the more practical choice if you often travel by taxi, because it is easy to carry and fit. Check the latest GOV.UK guidance on child seats in taxis for the rules.
The bottom line
Both seats cover the same stage, from around 3 to 12 years, and both meet the i-Size standard. If the seat will live in your main car and you want Britax's added safety technology, go for the Kidfix i-Size. If you need a seat that folds away and travels with you, the Cybex Solution G2 is the more flexible option. Any questions about fit? Get in touch before you order and we will help you check.
