You can use a locking clip to keep the seat belt from slipping.
I think most carseats come with them--at least the brands I've used
(Graco, Sunshine Kids, and Peg-Perego). It's this thing that looks
like two sideways capital Ts joined in the middle.
Here is a link to where you can buy one, but like I said, it may
be wedged somewhere in the back of your car seats (or the car seat
base, if you're still using an infant seat for your youngest
one).
http://www.amazon.com/Graco-4045-Seatbelt-Locking-Clip/dp/B0002ADQOU