Hi Julie! With young children, I think the best plan is to experience Disney's Animal Kingdom in a half day, rather than leaving for an early nap and returning later in the afternoon. Since your children won't be tall enough to ride some of the attractions with height requirements, like Expedition Everest, you should be able to experience most of the other attractions at Animal Kingdom by early afternoon.
I recommend you arrive at the park when it opens and head to the popular attractions like the
Kilimanjaro Safaris first. Definitely make time to see the awesome shows like
Finding Nemo -- The Musical and
Flights of Wonder. If your kids love animals, they'll really enjoy the
Affection Section petting zoo. Once you've experienced those "Must Dos," you can spend some time strolling the animal trails, before heading back to your resort for a nap break.
I love helping you with all your questions Julie! Please come visit me again here at the Moms Panel if you have more. Wishing you a magical vacation!