If you're talking 24" wheel - and unless your 9-yr-old is exceptionally tall or short I think you should be - then I don't think you can beat the Giant MTX.
My kids have had a Giant MTX and a Dawes Bandit between them. They're both pretty good, but the Giant just shades it for me in terms of build quality and components.
I guess it would come down to your kid's preference, but you'd get change from £200 for either of them.
If you're buying now, then you might find some 2009 models still in stock, before the 2010 models (and their inevitable price increases) hit the shops...