Okay ladies...remember me?
Been there and my other kids just got used to having the child safety locks on. Both my boys are very impulsive. Dave is 6 and I just started trying to under the child locks again but I am ready to engage them again. I just go around and open the door. It is a pain but worth the safety.
I also would put him back in the 5pt carseat if he can't undo that. Dave used to reach around behind him and unhook his whole seat and roll around the car. I had to get a car seat with a strap that secured under the car seat to a bar on the underside of the seat. It took a bit of work to get it in but once it was in it stayed and it was safe. No more seat belt issue. I also had to make sure that the 5pt restraint we got wasn't easy to undo and was one that locked in place at the chest and not just a regular clip.
Until they can learn to be safe and can be trusted I would really go with the safest route.
Susan, glad you were able to drop back in...we haven't seen you for a while and you've been missed.
I have to agree with Renee...you do need to put the 5 point restraint back in place for everyone's peace of mind.