Besides the mythical NoteSlate and the Eee Note, you might consider searching ebay for a used tablet PC. A couple of years ago, we scored a Motion Computing M1300 (1200? 1400?) for about $250. It's kind of slow, but Motions especially are well made, and would be rugged enough for a kid.

As I wrote in another thread, you'd probably want to get one that requires a penabled stylus, not just a plastic one, so that palm rejection, things dropping on the screen etc. aren't an issue.

And while it's too small, the Nokia N810 would have also been something to consider as they're quite cheap now and you can get xournal for it.