Dreams on the Bitface

A quick question for anyone who may know: what's the best of the handheld game systems for an eight- — nearly nine- — year old boy?

For values of "best" that include robustness, flexibility and the ability to stop playing it when you're told.

I realise the last requirement may not have been implemented on any platform yet.

Oh, and preferably in a sub-stratospheric price bracket, too.