Well, it's a great idea, it would just be very hard to actually make. For one thing, boosters do much more than just lift the child so the shoulder belt fits - they make sure the lap-belt fits as well, and provide leg support so that the child isn't scooting forward so his/her knees bend and the edge of the vehicle seat (which displaces the lap and shoulder belts, of course.) Highback boosters also provide additional protection in a side-impact crash, as well as head restraints in vehicles that don't already have them.
While it's possible that a manufacturer could create such a bench that would be compatible with ONE van, it's unlikely it would be universal. Vans in particular are so different in how the seats are made and where the seat belts are placed that each one would have to be customized.
That said, there is a European company making something similar to what you've described. I can't remember the name, but I'm sure someone will come along and post a link.
Now, as for your current problem, if you can post which seats your kids are using now, their weights, and the year/make/model of your vehicle, we might have some suggestions for you. :thumbsup:
