When we take long car trips, I usually put both kids in the backseat, and The Neurotic Dog up front with me. Part of the reason is that way both kids can watch the DVD player. On our return trip from OK, The Big One asked to ride up front.
Kansas has pretty strict booster seat and riding in the front seat laws. Neither one of my kids has met the weight requirement before the height requirement. In fact, if I had gone by weight rather than height, The Big One would have been in the rear-facing infant seat until she was almost two, and a booster seat until the fifth grade.
I really liked this arrangement much better. Since about two-third of The Big One's height is in her legs, she was much more comfortable and the dog had more room to turn around in the back. Plus, we were able to actually talk to each other, rather than both of us plugging into some electronic device.