K.K.
J.,
Hi there. I have a 3 year old. And she was doing the same thing. We tell her that she cann't get out of her set till the car has been cut off. But we also had to make the straps a little bit tighter. They want them to be where that you can get 2 fingers under them. If there is more room then he is not safe in his set. You also need to talk to your Son and tell him how unsafe it is for him to be takeing his arms out of the straps. Also you may want to tell him that he has to show little sister how to set in her set. Or tell him that the car will cann't go with out him in his set the right way. And with his arms out of his straps that is not the right way to be in his set. Make it like he is doing something to help you out. I hope that will help you in some way.
K.