To get so stressed about my kids’ car behaviour

My son did this once and I panicked and absolutely lost my sh*t and screamed at him (he was 4). I didnt scream anything abusive or unreasonable but I needed him to understand that he was never ever going to do that again. I screamed that it was dangerous, illegal and that he was going to kill us all or someone else in another car and that we'd all be locked in prison. He never did it again. You obviously need to make this clear to him because you clearly havent.

It sounds like your 5 yr old is bored when in the car. What activities are available to him during journeys? Songs? Nursery rhymes? I spy? Or you could tell him to let you know when he’s seen a yellow car, a red lorry, a goat/dog/cow etc and praise him massively when he does so.
Google search for car games.

You could also tell him he’s got to check everyone is belted in correctly, including the adults and if everyone stays belted in he’ll get x amount of stickers on his chart.

I’m sure you know the law on this @theremightbesevennowso it’s very important to solve this.
Royal Society for Prevention on Accidents also has info.
https://www.childcarseats.org.uk/choosing-using/children-undoing-their-harness/

I drove to the nearest police station. 3 year old sobbed and begged to go home.

you need to stop being so soft. Its a major safety issue . Drastic measures are needed. Scream at them to sit down and buckle up, they will be shocked and wont do it again.sometimes you have to be the scary person

No . They are not safe and probably illegal

Amazon.co.uk : car seat buckle guard https://share.google/xP33I3ZRMkSQ9SXuk

This. Give them a warning and then deliver the consequences.

To be honest, mine would have had an absolute bollocking well before now, but I have a big age gap between mine so I appreciate that I don’t have experience of dealing with two who are close in age and runs each other up. It’s clearly harder than dealing with one 3/5 year old.

Your 5 year old is old enough to know better. If he messes about like this on a school trip he will be told off by teaching staff - as I saw when I parent helped on trips in infants. One boy who kept taking his belt off was not allowed to sit with a friend on the homeward journey . A teacher sat next to him which he didn’t like and I remember her talking to the child’s mum when we got back to school.

I would focus on your eldest. He needs to be told how very wrong this is and that he will not be going to (wherever) if he does not stop doing this. Then pick a great activity, set off - and turn back and go back home.

It's illegal to modify the buckle with a lock. It must be visible and easily accessible. The AA did a full summary some years ago.

If they undo it you need to stop immediately with your hazards on wherever you are. Continuing at all is unlawful with an unrestrained child.

I would be giving some consideration to not taking them in the car at all and it if you do only one at a time. They sound like a hazard to themselves, you and everyone else on the road!!

These are illegal in the UK

You are an absolute duty to stop the car if they release their seatbelt. Hazards on and just stop.

We live in a rural area and I do just pull over. Sometimes we are a while, sometimes we miss stuff. It doesn't matter because they both now know there is absolutely zero way I will drive if I'm distracted or if they're not buckled in. No way.
I think you need to go zero tolerance of it. I also saw the police pull a family over with two kids unbuckled and scrapping in the car, I don't know what happened but you're at risk of getting a super serious fine for it. As well as obviously the risk to all lives including other people's around you.
I'd sit them down before going somewhere, calmly explain "if there is any fighting, shouting or unbuckled seats we will either stop till you behave or immediately turn around and come home. If we make it safely here is a reward you'll get whatever they love. To help you, we will play an audio story and play a game in the car". Then absolutely stick to your guns. You cannot drive if you aren't safe, you just can't. They won't believe you on safety if you don't stick to that.

No but you can set off earlier.
Plan when you are going to start the new regime so that if you have to go back home they are missing something they like eg a trip to soft play or somewhere rather than starting on your way to work.
I know exactly what you mean about narrow lanes, and passing places, I live rurally too, but surely you have to stop and fasten them in again anyway you can't carry on with them unrestrained.
These children know that they have the upper hand in the car, you need to flip that around.

Put top over head without putting their arms in sleeves, tuck sleeves behind backs then buckle them in firmly.

It will take the little sods longer to get free.

OP you said it is more likely on the way home: Are there serious consequences when you arrive? I think you really need to go OTT so that if they misbehave on the way home they will be anxious about the consequences. It is for their own good and they will remember whatever you decide is the harshest punishment.

A pp said this a few pages ago, and I can't find it now but I think she's right.

You're looking at the car problem in isolation but the root of the problem is that the kids don't do what you tell them. Well behaved kids want to be praised by their parents and are afraid of disappointing them. Yes, afraid. They should know that you'll follow through with real consequences and life will be grey and boring unless they behave better, and it'll be happy and fun if they do.

You are probably being much more permissive in everyday life than you realise.

That's not your fault, I think it's partly because families have become more isolated than in previous generations (post lockdown etc) so you don't get a feel for how strict other families are being and there's a vague feeling you ought to be gentle and smiley all the time.

But this car behaviour can't have come out of nowhere.

This, it is also this.

My kids knew from about age 2 that they had to do as they were told. It has to be ingrained early and reinforced with constant, consistent discipline. Sometimes you enforcing a boundary will mean they kick off or cry, yep thats life you keep on being firm. Or you end with 14 year olds who havent learned to mind & respect you, and ignore curfews etc.

I had this with mine when they were little. We were on our way to grandparents house and I pulled over in a parking bay and turned the engine off. Didn't say anything just sat in the car and waited.

That’s sounds so stressful, mine don’t unbuckle but they are both incredibly loud and this is amplified in the car, especially if they fight. I do have to just pull over until they quieten down because I get so distracted by the noise, it’s dangerous. I’ve also turned around and gone home before on particularly bad days. My older child is getting quieter as she gets older, fingers crossed for the younger one.

What's she meant to do with the other if she just takes one

Let them fight. It won't kill them. Unbuckling their seatbelts might kill them. That has to stop. Do whatever it takes even if it means taping mittens on to them.

This

And you can’t move one to the front unless you deactivate the air bag in the front passenger seat which is not possible in all cars.

Obviously don’t do this.

You can buy extra seat belt buckles. Speak to a car seat specialist shop like car seat ninja.

I think it's probably rather common. Children don't reason like adults.

I got a product called Houdini Stop which was recommended on Mumsnet best buys. A specific product existing suggests there's a sufficient market of people who'd buy it.

Prevention and cutting journeys down to the minimum were the only things that worked.

Not the cheapest option, but you could try tonie boxes and headphones. If they're more badly behaved on the way home it may be due to tiredness and overstimulation so listening to a story might calm them down a bit?
The stopping strategy does work well but I can see the issue if you're half way down a single width road with high hedges in the middle of the Cornish countryside.......