Instead of trying to teach animation I started with the idea of making a soundtrack for a haunted house. I asked them if they could scream, groan, and work out some grisley sounds; they could and did, and loved it.
We never actually got around to making a movie for the track to go on, but did quite a bit of other noisy recordings, and in the process released a lot of pentup tension in the group.
With young children it's a good idea to start with animal sounds. You can find many sites with these, but a good one is http://www.davisfarmland.com/fun/funsound.htm
Of course, all children can make animal sounds, but it is interesting to know what sound they make.
Trying typing in Animal sounds + Africa and then change the country.
You will find how different the animal accents are around the world at:
Another site at http://www.findsounds.com/types.html has a huge library covering just about anything you can think of. Acting out movements with the sound effects is good fun.
Another interesting site is http://freesound.iua.upf.edu/tagsViewSingle.php?id=181_3618
It is a good excercise to try and create your own sound effects with your voice, and things you can find in the class room. Shake, rattle, and roll things around; tap, drop, and scrape things. Then suggest how one can make sounds like the wind and rain.
UNESCO offers a free digital training kit at: http://unesco.uiah.fi/ydc-book/ which contains both audio files and an editor. Also http://audacity.sourceforge.net/about/
Here are some other sites to get started on: