To get so stressed about my kids’ car behaviour

Kind of even better then. Accept that (even if you are desperate to get home) pull over, plug earbuds in and listen to a podcast until they calm down. Plan your routes home differently?

Explain to them you won't take them out on n8ce trips as they cannot behave...you you are restricted to places you can walk to?

As PP suggested, show the older one the crash test dummy video.

Have a stash of entertainment to give them for the journey home (story CD/kids music).

You do seem to be very passive.

A 3 year old and a 5 year old don't have the capacity to understand a police person in the same way that an older child might. Especially if they don't obey their parent.

If they start playing up, all you can do is stop and not go again until they behave and/or are strapped firmly back in again. If they don't behave you turn and go back home.

Presuming they're both forward facing, I'd be getting an ERF seat for the 3yr old - facing in opposite directions might distract them from each other. Mine are 6 and 3 and can't really see each other as the 3yr old is rear facing. The besafe belt collector is also (I think - may need to double check) approved in crash testing to keep the buckles closed. You can also try supergluing some of the rough bit of velcro to the buckles - makes it much less attractive for them to press down on! But remember and keep a glove for yourself in the car

You can get seatbelt guards for normal seatbelt style buckles as well as harnesses. So get a seatbelt buckle guard.

Or are you not actually interested in doing anything actively about this?

Agree that it sounds like the 5 year old needs to be scared a little, possibly through talk of the police.

And you have to stop if he unbuckles himself or his brother, regardless of where you are going. Perhaps a detour to the police station if he doesn't get the message. I'm sure one of them would actually speak to him.

Have you tried the absolute nuclear approach.

show him the crash test dummy videos of a child flying through the windscreen and tell him he is risking his brothers life.

this isnt a situation for explaining. Death is finite - there’s no second chances. and he’s risking it for him, his brother and you.

You are coming across as rather resistant to the suggestions people are making, OP - you can't not drive, you can't just turn round, you have to go out, one DC doesn't heed consequences for doing something that is a threat to the life of their sibling.. And your child should be in big trouble for shouting at you, if they aren't already.

There's a different way of looking at this, and that's to flip from 'it's all too difficult' to 'This is a battle I'm going to win because I'm the parent.'

At 5 and 3 they should both still be in multi-point harness seats, which are harder to open and you can put locks on like a PP linked to.

The 5 yr old might be tall enough to be on a buckle booster, but if he can't be trusted then he goes back into his proper car seat

Just plan a couple of really exciting trips for them (not chores or paid for so it doesn't matter if you don't make it.). Tell them they have to behave in the car or you will have to come home. They kick off (even slightly) turn around, come home. They undo their seatbelts, pull over as soon as you can safely, sit until they do them.up again, and then turn around and come home. Repeat until they get the message..fucking around cuts the journey/trip short.

If it helps, open a letter in front of them, shake your head and say "oh no, you behaviour has been reported, we need to make sure you are safe in the car or we may get fined by the police".

https://www.amazon.co.uk/seat-belt-buckle-guard/s?k=seat+belt+buckle+guard

I found a good emergency brake stopped the unbuckling. Don't go to fast when you do it though as they literally do go flying

You're ignoring all the posts about getting locks/attachments for the buckles so they can't be opened.

These remind me of the pillow wall my dad had to builddown the middle of a double bed my brother and I had to share on holiday. Too much kicking, too little sleep on the first night.

What are the consequences for the DC5 for unbuckling the DC3, and how many times has it happened?

This makes sense. We sound similar!

No warning is something I have used only occasionally - 10-year old huffing and puffing about sweeping the floor of holiday apartment after lunch, no beach that afternoon; not heeding four requests between 1m and 4am to keep the noise down on what DD and best friend liked to call a 'no sleepover', no sleepovers for a month.

My tactic would be to head somewhere they would be keen to go to (and be fine with knowing you're probably not going to end up going there). Warn them beforehand if they misbehave in the car it not's safe so you'll need to turn around. If they kick off as expected head home.

Yes you can refuse to drive and you must. Otherwise you ate giving all the power in the relationship to your children, which isn't good for them. Children need to know that parents are in control, in order to feel safe.
Build in stopping time and be consistent about stopping and buckling back up every time either of them unbuckles their harness. They are a bit young for delayed consequences, so just give the one warning "If I have to stop again, we will go back home and you won't be able to go to xxxx".

@IdaGlossop I think I'd be saying one time before we leave the house that that will be the consequence and then following through immediately with no further warning. Then when we get home there'd be some sort of further consequence so that being at home wasn't the 'fun' option. But that's me.

@theremightbesevennow I think that dad made his own out of cardboard but there are also a few you can buy made of various materials.

https://www.splitandsit.nl/en/

https://amzn.eu/d/08j2YYgL

Might not make a difference if they're just going to ignore you and destroy it though tbh. Feel like getting a handle on them staying in seats first is gonna be a key step to success.