Ok. Thanks a lot for this input! Very helpful.
Accutally the children will receive brand new iPads within the next weeks. So the Pis will only be needed to run Sonic Pi (and as Robin suggests, possibly fluid- resp. qsynth).
I just think a Raspberry Pi can give you the feeling that computer technology is something you can touch and mess around with. Totally different compared to shiny and closed Apple stuff (as I said, they also have a few iMacs or iBooks where SP potentially could run). I think it is paramount that we not only raise new clients for big companies but rather show that technology can be molded and creatively appropriated.