I bought my kids a VTech MobiGo - sadly, compared to the games available on the family Wii, and my phone/tablet, it's only mildly diverting, and my kids aren't so easily amused that they didn't tire of it in a matter of days. I'm not making that mistake again (kid-friendly tablets aren't cheap) - when I think they're old enough for a real computer (or tablet, or whatever), I'll get them one, and make sure that they undetstand how it all works, and how not to bankrupt Daddy. In the meantime, I'm very grateful that every purchase through the Play store requires authorisation (does that inconvenience you dreadfully?), and that my kids are smart enough to understand that they are not to attempt "real money" purchases, anyway.
[Edit: I seem to have wandered into the "speaking as a parent..." trap, and thus probably invalidated my own opinion. Bugger.]