R Yogi:
Thanks for coming to visit us at the Moms Panel.
If you've got little ones needing a nap and you'll be going to parks other than the Magic Kingdom, then yes, I would recommend renting a car for those mid-day trips back and forth to the resort. If your only park were the Magic Kingdom, it'd be a toss-up, as getting to your car can take as much time as waiting on a bus at MK.
Since we're on the subject of naps, I would highly recommend that everyone come back to the resort. There's a lot to say for a break by the pool or just resting in the room so that you can get the most out of the return to enjoy the park in the evening.
Last thought: If the rental car negates your savings of choosing the
Art of Animation resort over the Contemporary, you may want to revisit your resort choice. But my intuition tells me that it might be a good expense for the trip because it will also allow you to go to the grocery store for incidentals that are difficult to pack on a plane like baby food, diapers, and even strollers. Best of all, parking is free for resort guests both at your hotel and at the theme parks.
Wishing y'all a magical vacation!
Whitney