I see you're trying one of the pro versions. Any of the pro versions should work. For that matter, I think most of the current consumer versions would probably work. I know the Sony Vegas Movie Studio does chroma key, and allows 4 video tracks and 4 audio tracks. It's difficut to find someone who has significant experience with several different brands, in order to compare ease of use.
I'm using the pro version of Sony Vegas, doing lots of chroma key, and find it easy to use (though of course, there was a learning curve). I have seen magazine reviews that say it's relatively easy to learn. (It's the only one I've used, so I can't compare.)
If you can find a store that sells academic-discount software, your kids might qualify for a significant discount on the pro version of Vegas or Premiere. Try
http://www.gradware.com and do searches for "sony" and adobe".
Good luck, :)
Ken Hull