Harness AND seatbelt for 4 yo?

[Albta]

i think this might be a stupid question…
I have a maxi-cosi 123 car seat, isofix, ff with tether.
My child is a tall 4 year old and according to the instructions I should be using a seatbelt with the car seat now.
Can I use the harness AND the seatbelt?

If your child is over 18kg it is dangerous to use the harness.

Thank you - but why is it dangerous to use the harness?
I think it’s not strong enough, but if you use the seatbelt as well - surely the combo is safer?

The risk of decapitation in an accident is higher once they reach 18kg.

I'm not an expert, but believe it's because of the shoulders being pinned back but head able to freely move forward.

No you can't use both at once. It's one or the other. Once you move to seatbelt, you also usually would stop using the top tether. Double check this but I believe that's what the instructions say. There is also something which needs to be done to the headrest to release it from the harness and allow it to move up further as your child grows.

I assume he is over 105cm - are his shoulders still under the headrest/top harness setting, and is he within the weight limit of the harness mode? Does he weigh more than 15kg?

Do you think he'd be sensible enough to sit with the seatbelt? A lot of children are at 4.

No seat on the market (apart from some SN seats) allows both to be used together - I think it's to do with how the forces in a crash would be spread. Harnesses/seatbelts don't only hold you in, they also spread the impact force over the strongest parts of the body. If you've got a harness and a seatbelt lying over each other then they could interact in potentially dangerous ways. If the seatbelt is on top then it could be pressing hard parts into the body like the buckle, if the seatbelt is underneath then the harness is possibly moving it and changing where it lies on the child's body. Plus you then have a really confusing situation if you were (God forbid) in the situation that a stranger/rescuer, or even you in a panic, needed to quickly free DC from the seat. If they undid the wrong one first it could get all tangled up and cause a hazard.

It's not dangerous to use a harness over 18kg, if the harness is rated higher (e.g. Joie Bold, Axkid Minikid.) It is dangerous to use a harness with 18kg limit at a higher weight, because it may fail. All the Maxi Cosi 123 seats have 18kg limit IIRC.

I am not sure what the decapitation post is about (sorry) I think maybe this has got mixed up with something else? FF vs RF risk?

No, you can't use both together. Ask the manufacturer.

@BertieBotts but the OP is talking about a FF car seat? Is that not correct then?

The risk of internal decapitation is higher when using a 5 point harness FF. This is because in an accident the harness holds the body in the seat and the head is free to forcibly move forward. The small bones of the neck in children under 5 aren't mature enough to withstand such forces, and so this can cause catastrophic damage to the spinal cord. The head of a young child is also significantly heavier relative to the rest of their body, compared to an older child / adult. At 5, providing the child can sit sensibly with a 3 point adult seatbelt, this will be safer as it pins the body less. A child of this age is usually mature enough to sit sensibly in a HBB.

ERF seats have 5 point harnesses, but in an accident the child would be forced into the shell of the seat so the seat takes all of the impact, spreading the force. The only time RF with a 5 point harness wouldn’t be safer is if the vehicle was reversed at speed into a wall (for example), which is a highly unlikely scenario.

It’s always dangerous to use a harnessed seat over the weight limit, regardless of whether it’s a RF or FF seat, because it won’t have been safely tested beyond this limit.

All very helpful - thank you all!!
I will only use the seatbelt now!!

I’m going to go and check the tether and headrest instructions as well - the headrest is fitted for his height, but I’ll check there isn’t something else I need to do.

I’ve checked and it’s not set up with the tether when they are bigger.
@BertieBotts thank you for flagging - I didn’t spot that when I looked at the instructions first!

Yes she is, but she's talking about the transition from 5 point harness to adult seatbelt - there's no known change in internal decapitation risk with this transition that I'm aware of, and if there was I am not sure what the weight boundary of 18kg refers to.

Internal decapitation is extremely rare at 4yo, even FF.

The way to reduce the risk of internal decapitation is to use a rear facing seat for as long as possible, particularly for much younger toddlers.

The only thing I can think you might be thinking of is the idea that a seatbelt puts less strain on the neck than a 5 point harness? IIRC this turned out to be a bit of a myth (or at least, no strong supporting evidence) popularised by the companies marketing impact shield seats.