With so many kids I think I have one in just about every size. If I had to take everyone I have 3 that fill the whole back seat and I also have the console style. I love them and better yet my kids like them.
http://www.snoozer-dog-beds.com/dogcarseats.htm
I did get all of mine off of ebay but many companies carry the Snoozer Lookout Seat.
The cover comes right off so you could take it to anyone who does embroidery.