To get so stressed about my kids’ car behaviour

My rule when they were young and in that phase was 'I will tell you once to sit down, do your seatbelt up and stop squabbling, then I will pull the car over. If I have to pull over, it's smacked backsides all round and if we're going somewhere fun, not anymore, we're not.'

I only had to wallop them once and cancel plans a couple of times before they learned to behave. Playing up in the car is dangerous to the point we could all be killed if they distract or jostle me at the wrong moment. Absolutely zero tolerance on it did work.

Physical violence is never necessary and shows only a parent unable to manage themselves, let alone a child.

Well I'm sure the 3 year old at least is in a 5 point harness seat. My 4 years dgc is in a joie spin that faces backward with 5 point harness. Imagine more difficult to escape

That’s a great idea. Try to nip these in the bud early otherwise they’ll be enemies in war for a lifetime.

A sharp slap does wonders for exactly this situation

Pretend to take them somewhere nice (theme park or something). Then, when they kick off, turn around and go home. Tell them they can go once they learn to behave

Similarly, one of mine unplugged the harness when we were on the motorway once and I absolutely lost my temper with fear. He never did it again.

It's not a safer option if your eldest can unbuckle him, is it? You need a five point harness

I agree with you. It may also be that OP is driving her children down narrow rural lanes right now, stopping at every pull-in she sees. I'm being optimistic...

Tell them if they are going to not be sensible enough to use adult seat belts (I'm assuming they are both in high back boosters with the adult seat belt?) then you'll get them seats with five point harnesses like babies have (at their ages there should be ones on the market that they fit into!) Then say that if they have to have the baby seats, they won't be able to go in anyone else's car (if they do for playdates etc) as it'll be too tricky to move their baby seats into other cars!

Personally , I dont think OP wants any suggestions. She doesnt seem to have done much to control the situation. The kids are 3 and 5 they are little. Country roads have farm gates and houses too. You can pull in, no its not a parking spot like in an urban area but it happens. Its far safer to pull in than keep driving while 2 little kids riot freely inside a moving car on said narrow, bendy roads. Yes I live one some of these narrow roads in the middle of nowhere! I have one who gets travel sick. I have had to pull in absolutely everywhere. You figure it out. OP is asking for advise but replying that it doesnt apply because she is rural, because there is no link... excuses!
If its just sympathy OP wants well yes its hard. Its not fun but it is so very dangerous and you need to be their parent and stop it, regain control in the car. You have been given so many options here but seem very uninterested in them.

Plan something they really enjoy and travel on a safe route with lots of opportunities to stop and clip them back in. Give them one chance. If they don’t follow the rules you turn around immediately and you go home. Do this every single time until you break this habit. Tell them what will happen before hand. If they get out you buckle them back in and go home. No discussion, no explanation. Don’t engage and do not back down. It’s a bad habit but it can be fixed. You can do this!

This is one of those times you need to properly loose your shit with them. Pull over and let them know in no uncertain terms that you are not putting up with any more crap. No negotiation, no gentle parenting. They need to be a bit scared of your reaction if they ever try it again .

Mine are the same ages and have just started this nonsense - arguing over who puts their water in the big bottle holder etc. Yesterday my eldest undid his car seat strap for the first time. I'm not a shouter but absolutely lost it. Stopped the car soon as possible. Talked through consequences if there was a car crash), refused to leave until it had gone in. I've told them if it ever happens again, they will
not be going to any fun activities that involve the car again.

We bought an enormous panda bear and sat it between them. They couldn't see each other round him and totally chilled them out. They could lean on him to sleep. For years Charlie panda went everywhere with us. Putting one in the front created even more arguments.

Something like this ?

So tell them that the following Saturday or whenever, you are planning a trip to fantastic x place but any car shenanigans before then will mean it won’t take place. Should prevent returning home shenanigans and if it doesn’t they miss out on a trip.
or
Tell them after a bout of bad behaviour, that you were planning a trip this weekend (even if you weren’t) and that now you aren’t going. They will think they’ve missed out. Then say that depending on their behaviour over the next x time, you may consider going then. You could pencil in a date ahead and tick off each day of good behaviour. But be strict and follow through.

Wonder how your kids will cope if someone else shouts at them? Crunple into tears?

Not good enough when it comes to the risk that they pose to other people on the road.
We had a chest lock seatbelt for DS. There are options.
If they cannot behave in the car then they don’t go in the car, tough shit, some poor family will be burying their loved one because OP can’t control her DC or invest in seat belt locks.
If she puts one in the front, which isn’t safe with an airbag they squabble over who sits in the front seat.
Watch a few Super-nanny episodes.

Hear hear.

My cousins used to do this and my uncle once was so stressed by them so drove straight to the police station! Both cousins were still fighting and scrambling over the front seats so didn't notice. My uncle called a police officer over and asked him to have stern words with his kids! It actually worked, they never did it again, I'm surprised he didn't crash the car while he was driving. My cousins were really feral as kids.

A copy of Strewelpeter needs to be bought!

I get this, I had one who tried doing mad things in the back of the car and you can’t always pull over.

I recommend waiting until you don’t have anything on you have to go to for a couple of days and deliberately planning some drives on routes you know you can pull over safely. The purpose of which is solely to ‘train’ your children how to ride in the car safely. Then you can do the pulling over thing that PP’s suggest as many times as you deem necessary without the added stress of needing to get somewhere/on a dual carriageway because you’re only in the car to train the kids.

This is what I did. I was very frank about what I was doing- unfortunately, because you are so dangerous in the car, we are going to have to spend the next couple of days learning how to be safe instead of xyz more desirable activities. I will be looking to check you stay strapped in and not making lots of noise so I can concentrate on the road. We will keep doing it until you have learned to behave.

I went over the potential consequences of being dangerous in the car every time we stopped and explained he was putting himself and me in danger, so I had to stop the car. Mummy felt scared because we weren’t safe, so I had to stop the car. You need to look and sound scared/alarmed when you deliver the speech, or they won’t take it seriously (or at least mine wouldn’t!)

I imagine the consequences of behaving so dangerously might be lost on the younger one, but the older one is old enough to understand that a) the police could pull you over if they see children without seatbelts on and you’d be in serious trouble and b) if there was a crash, they would be seriously injured or killed. And being silly and distracting mummy while she’s driving could cause a crash because drivers need to focus on the road. Mummy and daddy will keep you safe in most situations but we CAN’T keep you safe if you’re deliberately putting yourself and the rest of the family in danger while we’re driving a car.

Damn, they are clever little B*s aren't they?

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.