Some people only allocate Animal Kingdom a half day, something I strongly disagree with. We always allocate 2 days at each of the 4 main parks and STILL never have time to see and do everything. So I suggest allocating one full day to Animal Kingdom, especially since it often closes at 5 pm, which is quite early.
There are many animals in several different areas, so your children should love it. I made a post in the following thread, listing all the animals that we saw (and where they were). Just scroll down to post #21 by Pigletlover:
http://www.disboards.com/showthread.php?t=992873&page=2
There is so much wonderful theming and detail and little extra things to see and do at Animal Kingdom that many people probably don't notice some of them. Read the rest of the above thread to find out about the talking palm tree, the human vine Devine, the talking trashcan, and the talking bushes.
Considering the age of your children, I would say don't miss these things:
Kilimanjaro Safari
Maharajah walking trail
Pangani walking trail
It's tough to be a bug (4D show)
Festival of the Lion King show
Pocahontas show (this has animals)
Flights of Wonder bird show
Rafiki's Planet Watch (you take a train there)
The afternoon parade
And make sure you take a close look at the big Tree of Life to see all the wonderful animal carvings on it.