I think the Kindle Fire, and possibly the Nook tablets come with kid timers. I know Apple doesn't make one, and I doubt apps that do that kind of thing would be allowed in the App Store -- keeping time, yes, but shutting down the tablet after a preset time, no.
But then, books didn't come with preset timers, either, and hmmmm, I dunno if I would be a better person if they did or not!