My son is three and a few weeks ago he figured out how to unbuckle the seat belt in the van. He's in a car seat carrier and the belts for the carrier are made child proof, but the belts for the van are not. He reaches behind him and all he has to do is push down on the red button. I never had this issue with my other two kids and I have told my youngest over and over, not to mess with the seat belt. I think he sees the other two unbuckling the belts when they get out of the van, so he feels he can do it at anytime.
I heard that one in every three children will unbuckle their seats before the car stops. And this can happen to kids as young as two years of age.
Here is what I did for a quick fix. I rapped a small towel around the Seat Belt Buckle and put some tape around the towel so his little hands can't reach the buckle. I am looking for a permanent fix to the belt buckle. I found a product online called Angel Guard. It will do exactly what I am looking for. http://www.angelguardbuynow.com