Much as I hate to admit this, I used a walking harness that fastened in the back, and tethered that to the back of the car seat. Even when mine would get the shoulder straps off, she didn't have enough mobility to wiggle out of the leg straps before I could pull over and stop her. Of course, I only had to do that until my daughter was three or four, at which point, I discovered the lifesaver of a second-hand car seat strapped in next to her with one of her favorite dolls or stuffed animals in it. For some reason, that worked well for us.

We used to Jacob Houdini b/c he could slip out of the 5 point harness! But the up-side" to Asperger's was that we could hook him in, put a velcro strap across the harness and told him it was his "super-hero" lock. He never tried to undo it. It had no function for safety, but he believed that it did. That was all it took!

I saw a guard of some type on line somewhere while doing research for the same problem, but it depends on what kind of car seat it is. At the time my son was already in a booster with a back and the guard was for where the seatbelt snapped closed. I wound up cutting up the top of a milk jug and hooking it with a rubber band to the clip part of the seatbeat...the bottom part that sticks out of the seats.

i have the same problem i notice if u Cris-cross the straps it makes it harder for them to pull their arms out. that is what has been working for us... also, the car seat we have, we are able to tighten it with a knob on the side of the seat so we can make it tighter every day to assure his safety! Good luck!!

You could also try applying for grants to help pay for equipment. Don't know if it would qualify but it would be worth the try.