I don't know how old your children are but mine are 4 and 2. They also used to do this until I was honest with them. I tell them the police will yell at me if they are not buckled right. And, if something happens to them I will be very sad.

I think it also reinforced it when they had a safety talk at school recently.  They don't ever get out of the straps anymore. 

pass a baby shop also just see if the ajustments are right for him and they also might have a few ideas as to what to do about it or recomend something to buy to help you with this ,,,

As stated, you can buy things that strap to the top and they can't get their arms out. It clips at the back, on their body-side of the straps to stop them undoing it.  I used to pull over, smack her leg, tell her NO, then put her arms back in and drive off again. I did it 3 times in as many days, then she learnt and has never done it again. We had a little girl staying with us when they were both around 2, the little girl took her arms out while we were driving every time.  I couldn't smack her, so I had to simply leave time to stop 3 or 4 times on the way to wherever we were going and back again. I knew I didn't have to live with her so I put up with it for that time.  Now my daughter still tells everyone that her friend took her arms out of the carseat.  My daughter is 3 and a half now and I haven't had a problem since she was about 12 months old.  It only took 3 little smacks on the leg for her to know not to do it again.  But you can buy those straps that go across their chest between the carseat straps that keep their arms in.  I wouldn't recommend a tie because in an accident you need to get him out in a hurry, not stuff around with ties that can knot.  Good luck with it all.

You can actually buy a piece that you can add to the seat belts. You attach it up near there chest that way the top cant be separated. They work well. You can buy them in places like Kmart or Big W. Hope this heps.

my son undoes  his car seat belts and he is 13 months old and has been doing this since he was 9 months i have to put things around the belt buckle. but maybe a hanging toy or one that is attached to the starps for him to play with or you could use those looney tunes shoulder strap things that go on the adults seat belts it may keep him busy?