We use a roof rack on our van. It's out of the way and safe (as long as you remember not to drive into the garage!), but it's a pain in the neck to get them up there. I wouldn't mind trying transport them inside, but that's usually not feasible if I'm carting around the twins and all their accoutrement.

If we hadn't had the twins, I'd still be driving my Audi station wagon. God, I miss that car!