Anne:
You will not miss with either option. I love
Sanaa, and the outdoor deck outside the restaurant from which you can enjoy the animals provides an up close view of some breeds you may not see at Animal Kingdom. Make sure to ask for a window table to look out for even more animals. Their menu is more adventurous than the buffet at Tusker House, so if you have picky eaters, make sure to go online to check out its menu before your final decision. Keep in mind that there are some old standbys like macaroni and cheese on the children's menu.
If you think you won't arrive early to the parks, Tusker House will allow you to stay until park closing. It will probably be more expensive than your bill at Sanaa, so if you are on a budget, weigh this in your decision. The gain of time may not be as much as you think, though, as you will be sacrificing time out of the park at noon rather than at the end of the day if you were to leave for Sanaa.
If I had a choice, I'd head to Sanaa. The children will be able to view even more animals from the resort's viewing patio and you'll still end your evening early enough to be rested for the next day.
Wishing y'all a magical vacation!
Whitney