My son, who is just 8, has a Trek MT60, which looks like a generic kids MTB, but has a lot of features such as smaller grips, kids brake levers, adjustable stem (height and reach), two pedal positions on the cranks, dual chainguard and so on. He got it just after a trip to Center Parcs where he had a hire bike that was nominally the same size. The difference is remarkable - the hire bike he really didn't like, he couldn't get up the slightest hill and if it started going a bit faster he felt unsafe and had to brake. Got him on the trek and he was away, standing up on the pedals, pedalling it up hills, racing along the downhills, fantastic!
It does have an aluminium frame, but it is quite heavy, although that doesn't seem to worry him. It also wasn't that expensive really - more than you might pay for an Apollo, but worth it IMO.
Colin