DS lites are 109 on amazon, DSI 130-140, 3DS 180ish, games vary in price i imagine with new ones starting at 30. I believe dsi and 3ds can download games.
Ipod touch 4 starts at 190, ipod touch 5 starts at 280, lots of free games and apps, and the ones that aren't usually only cost a buck to five bucks or ten.
Nexus 7 and Kindle fire start at 200, lots of apps and games (although not as much as Ipod) free or a few bucks. bigger screens that any other device in this range.
Something to think about for older child, if they don't have a laptop yet.
I would think this might help in future with school work, but the kids might want something pocketable. Pretty cool though.