My 11 year old son is riding my Kent Ultralite now, and likes it pretty well. And I heard you can get one with a three speed hub now, which is a big plus. But he prefers my Downtube Mini, which is simply a better bike. My 9 year old daughter can also ride the Mini but it's a bit large for her (she's a peanut); even so, if you're looking to buy something, there's a lot to recommend a good quality 16" wheel folding bike, i.e. either the Curve or the Mini. Not the Kent; the quality is just too low.
I believe Raleigh made child size folders, like an RSW-14 or something, and later a version of the Twenty with proportionally scaled down wheels... if you can find one of those, that would be really cool. And of course there's the Moulton Mini, if you can find one.
+! RHM
I forgot the Downtube Mini great bike with more versatile gearing.
If on stricter budget then the Kent.