To get so stressed about my kids’ car behaviour

You have to set up a fantastic treat, that you know will probably not happen. Ideally on a route where you can stop easily.

Tell them we are going to wherever, but you MUST behave in the car, and the minute they start the nonsense, turn around and go home. Don't even wait for them to get the seatbelt off, so it is safer to turn around!
Have something ordinary planned for the day so you are not disappointing yourself with the lack of the day out.

Been there done that. I am sure not everyone will agree with this tactic but it worked really quickly with min
Get ready to go somewhere they really want to go-park/friends/seaside
Give them a warning before you get in the car that if they misbehave just once you will immediately turn the car around and drive home. Make it clear what is and isn’t acceptable-no unbuckling seatbelts etc.
The most important point is-follow through with what you have said-no second and third chances, turn around and go home.
Calmly explain that they were told not to do xyz but they did it anyway so that is why they are no longer going to park/friends/seaside.
Rinse and repeat every time.

This is the only option . My ND kid never listens to me but they tried the seatbelt thing once and I lost my mind as it was so dangerous. Never tried it again.

Couldn't you use something like this

https://www.amazon.co.uk/Barrier-Adjustable-Divider-Storage-Pockets/dp/B0H4Z9LGGC/ref=sr_1_44?dib=eyJ2IjoiMSJ9.N3j2cCi6aLWo0ze1HMmSHhp4rxgd6Hp65sbttVO0Qe7Atht9pIXyGGHisi2VwedWlKs79_MN9eviI8IushyC-m1qJHkWsggsiP1rZ5wOLT-zmOuAKlBY0SKyvG9Z5GnypTuS-widDWQS_vJxx1IUwj0pK6DommOgrAek33mRROl62s8t1nMsUElgMyPW0-0TdPTtaOcjO4pZv0-_9POc65ZTThkrxxqrK0f9UCpPB93xJ-caKep4ZwAJXJBbq2RYZaIg0GTC2USYs5m6mkm8RrgSZoizDlzoT_IsN04uh5o._i7wGizqUZ43ZoaL0xYYx7cco-CmldU-6000tAwPQ9s&dib_tag=se&keywords=car+seat+divider&qid=1786174359&sr=8-44

You could attach it to the rear headrest & round the car seat base buckle, then attach to the front seat? Then neither of the kids could unbuckle the other as they can't get through the mesh?

Crelling Harnesses might work for you here.
Wont stop the bickering though

This is good advice.

When my ADHD son was about 4 he went through a similar stage - I lost my shit at him and then noticed a police car and told him they were coming to arrest mummy because he wouldn't keep buckled in 🙈

My way worked but the advice above is better!

I'd put the older one in the front tbh. Then you can see what he is doing, they can't fight, and he can't unbuckle the younger one.

I'd also start absolutely losing my fucking shit over it. We had a phase of this with one of mine, which rumbled on for a while with me patiently explaining it wasn't ok, and stopping the car when it happened, etc etc. But I am not ashamed to say that when my child's leg suddenly appeared in the gap between driver and passenger seats trying to climb into the front while I was on the fucking M5, I yelled at him GET BACK IN YOUR SEAT RIGHT NOW so loud my throat hurt afterwards and slapped said leg quite hard 😬 Obviously I do not recommend that as a parenting strategy. But that, combined with the huge bollocking he got as soon as I stopped the car at the next services, did the trick and it didn't happen again...

To add. A little horror story of truth is okay in these circumstances.
I told DS about an imaginary child who fell out the upstairs windows, her head cracked open like an egg, she had to leave her family to go to heaven. He stopped attempting to escape out windows.

No need to shout - I have never shouted at my children and I don’t allow them too either

This isn’t an option. It’s extremely dangerous. Buy an extra lock and harness for the car seats. If you’re going somewhere fun bring them straight home. Punish them for bad behaviour once your home, enough to know that you mean business.

There's nothing like an emergency stop to make kids pay attention. From a slow speed, and with no other traffic around, but that should be easy with rural roads. An unexpected physical jolt combined with a bollocking is an effective shock.

It sounds like the bickering and dickishness comes before them taking their belts off: don't let it escalate that far. Pick them up on the attitude the moment it gets too much.

Leave early and stop each time they misbehave or go straight home and no screens etc as a consequence

Also, worth mentioning that I'm experiencing something similar with my 8 year old - lots of his friends are out of car seats and he isn't tall enough yet. I firmly explain that it's the law, it's out of my control and he cannot travel unless he's safe. And we don't go unless he's willing to comply. He has a moan, but grudgingly accepts it. Most 5yo's can understand this reasoning.

Hazards on and you slow right down, stop as soon as yiu reasonably can. Just like you'd do if you had some other safety issue e.g. bakes issue. It's no different. Sounds like where we live, even the narrowest lanes have passing points, farm gateways etc which you can pull into for an emergency.

Totally unnecessary.

My kids are coming up 5 and 3 earlier this year my dd started this too and I knocked it on the head asap.

the kids know once the car is in motion you pack it in and behave.

You just need to be really firm and make it clear dangerous behaviour has consequences.

Anything fun or optional like weekends....
before hand you explain what's going to happen. And you explain you could all endu up in hospital or dead.
If they do it ypu pull over rebuckle and have a stern word. The start to proceed home.
At home no tv, no sweet treat and a hair wash at bedtime for good measure.
Post feed pre bedtime rankings you say - i dont want you dead or injured this will happen every time now.
Tomorrow we will try again.

For mandatory trips - pre trip pep talk, nice music and a bribe. The classic "chocolate button" or mini marshmallow works wonders in our house.

For you yourself try not to get stressed and get a grip and enforce safety

You need to address this asap - you are going to kill or injure someone otherwise.

Absolutely bollock them and scare the hell out of them.

Parents who shout rarely have obedient children. Work on actions and consequences. If you shout when they are shouting that reinforces on them that shouting is ok.

Leave the house much earlier than you need to to have enough time to pull to the side and stop this fucking nonsense before you crash or runover a cyclist or pedestrian.

If you cannot control them when in the car you are endangering other people big time.

So you just carry on driving with a 5 and a 3 year old unbuckled and moving around the car?! You absolutely must pull over and rebuckle.

The only lie there was that you threaten a visit to the police station that you knew wasn’t going to happen but all the rest is spot on. Good call.

Turn around and go home. The second they start, turn around and go home and if it carries on on the way home then so be it. The next weekend you refuse to take them out because of their behaviour.

I would start back at the beginning for a little while.

Don't have them in the car. Take them for a walk instead. Show the dangers of being at the side of the road and how to keep safe. Whilst doing this, discuss how it's everyone's job to help each other stay safe and we must do this calmly. These are the rules for staying safe whilst walking. Get them to look out for buildings and trees, birds... whatever!

Do this a few times.

The same then applies to the car. It's everyone's responsibility to help you do safe driving. What are the rules? Can they find a blue car, a red car, a truck or tractor?

Discuss a consequence to the behaviour.

If they do unbuckle or break an important safety rule pull over. As soon as you can. "Seat belt on please.".. thank them...then drive off (you may need to actually belt them up).

When you get home, the consequence is communicated along with an explanation.

I hope it helps.

Surely just invent a lovely trip somewhere? the zoo. Wherever. Then drive in a direction without winding country lanes. Then turn the car round and go home when your eldest DC unbuckles your youngest? Sit on your driveway. explain why you've come home. etc etc. Also sit down with your DC5 together with your husband & show him safety videos on the importance of seatbelts. ring your local police ststion and ask if you can bring the kids in (eldest only maybe) so they can explain why everyone needs to wear seatbelts. You cannot allow this behaviour to continue OP!

I'm generally a pretty gentle parent, but I would absolutely roar at a 5 yo if they were undoing their younger siblings seatbelt in a moving car. 5 is old enough to know better. And yes consequences!! So going home immediately if something fun or yes, put them to bed! Angry face. Telling dad when he gets home. Dad also has a STERN talk with them. Enough that he will remember every time he thinks about doing it again!

I wouldn’t let either of them in the front, however I’d consider if there are any local community police (PCSO) who could come and talk to them about car safety and that if they do it again they will take mummy away to prison?

A smack beats death in a car accident, hands down.

Show the children videos of those crash test dummies. That's a clear visual of what happens