My son could loosen them in his Marathon. Fortunately, he didn't realize it until he was a little older (closer to 3, I think) so we talked about safety and how he was never allowed to touch the straps when we were driving. Anytime your son does this, you need to tell him it is unacceptable and immediately pull over to fix it, no matter how often it happens. He will lose interest in the game if he realizes that you're never going to go anywhere fun if you have to constantly stop to fix the seat. Also, when you do, don't give him any positive attention. Just fix the seat and sternly say "No. You cannot touch the strap. It isn't safe."