HBB for big 7 year old with anti submarining clip?

[SingingWaffleDoggy]

I have a tall 7 year old (135cm in age 9-10 clothes) who has been in a Axkid BigKid 2 for the past 2 years. My DH has now broken/ snapped the polystyrene side leaning over to seatbelt a friends child in and to be honest the fabric cover keep coming off anyway.
One of the things I liked about it was the anti submarining clip, as I found it reassuring going from RF to a HBB with one. However, the gap between the back of the seat and the clip wasn’t quite deep enough so she hasn’t fit in it for a while now and has just been using the 3 point seatbelt without this extra.
I will occasionally need to use it for my soon to be 5 year old (currently harnessed in a Radian 5 which seems to be the most bombproof car seat ever!) so I think I would prefer another with a clip. I suppose my question is actually 2 questions:
Does having a submarining clip actually make it safer or does it just make me feel more reassured? Is there evidence?
And if it is safer, which HBB with one would be most likely to fit the big kid?

submarining, like underwater?

There's no evidence they actually help - they might even adversely affect the fit of the lap belt. The idea that they reduce loading by X amount seems to be based on an assumption that the lap belt would be on the abdomen without the clip - so a good position of the lap belt across the pelvis is what you're looking for.

That said, if you wanted to try one, I would look at Besafe izi Flex Fix - I think that's the biggest. Not sure if it will be far enough forward for a child the size of a 9 year old but you could see if you can find one in stock somewhere to try.

If that is no good, Britax Kidfix Pro recently got the best score ever for a HBB on the ADAC test, and that has one and Axkid Up did well too, and is meant to fit both younger and older children well (but is very expensive, and no lap belt hook) - or you may wish to prioritise a seat which will last as long as possible? I believe Avionaut Maxspace is the HBB with the highest shoulder belt guide so will last the longest.

In this context the term "submarining" refers to a type of injury seen in young children who are in car crashes when using a seatbelt without a booster, where their hips can't correctly anchor the seatbelt, so rather than being held in place by the seatbelt, the child's bottom slides forwards under the forces of the crash, which means that the seatbelt ends up across their tummy and neck which leads to injuries.

Some high back booster seats include what they call a fourth point of attachment, which is essentially a hook or loop which goes between the child's legs and attaches to the lap portion of the seatbelt, holding it in the right place and supposedly preventing the child from sliding out under the belt.

It's not strictly necessary, because submarining is not an injury commonly seen with children in booster seats, provided the booster seat is properly used. You see it sometimes if the booster is misused e.g. seatbelt routed over the belt guides instead of under, or a poorly designed booster which routes the seatbelt too high. However, many parents find the clips reassuring especially if they feel anxious about moving to a high back booster, which is why they remain popular.

Hi OP,

The Avionaut Maxspace is meant to be very long lasting and great for tall kids. It doesn’t have the lap belt guide but I understand that they are much more beneficial for younger children. Having said that, we have a BeSafe FlexFix 2 which has the guide and our almost 8 year old still uses the it. A crash tested footrest would be great idea though, especially for your younger child. We have the KneeGuards footrest, but I believe Swandoo make a similar one for the same price.

Thank you for your help. We tried the be safe flex fit which was big enough but didn’t look comfy, so then tried the Avionaut and it was a much more spacious fit with lots of room to grow.
We tried the little one in it as well and the belt sat nice and low so I do feel much better about not having the extra strap and the lumbar support also makes sure they don’t slump so I’m very happy with it.
Thanks again!

I ended up going into a deep dive on this the other day after the post.

What I found out is that the clip things are in fact considered effective to support the right lap belt position, but they aren't the only way to support proper lap belt position and other features of seats can also do this such as the position of the lap belt guides, the angle of the seat itself and some other things.

So I was mostly right and my conclusion that they aren't necessary was correct, I was just wrong about them being ineffective. They are effective, but they are not the only way to promote good lap belt fit. The concern about them is if a seat manufacturer designs a seat where the only anti-submarine feature is the clip, you essentially lose that protection if a parent/caregiver uses the clip wrong.

Anyway - as said the positioning in the Avionaut is very good and would provide an anti-submarine effect on its own despite no clip