Mate it would be FAR cheaper and FAR safer to leave the kids in proper 5 point harness. You can get them to accomodate up to 36kg which is getting to 7 or 8 years old. Booster seats, even integrated ones, simply aren't safe enough for kids.

http://www.youtube.com/watch?v=azgBhZfcqaQ&NR=1

When I have kids, they'll be in 5 point harnesses as long as possible!