rPi might seem ok at first, but I'm really not sure it's the best solution for cases like this, I find it's too much of a 'hacker' device? Sure if it works, it gives you a pretty usable general purpose OS (kinda slow though) but if it breaks (and it will sooner than the ordinary laptop/PC, either in software or in hardware) you'll have to have quite some linux experience in order to fix it.

I agree with your last sentence though. One single device tied to a single OS is a bad choice. I'd rather give them a laptop that dual-boots Linux and Windows. That at least gives children a taste of the real world where more than one OS is used and learns them how to use both.