I've just been doing the 24 inch wheel thing for my son's 9th birthday.
I've stuck him on a number of 24 inch wheeled bikes, including the Specialized Hotrock, but I've been somewhat disappointed with all of them. The hotrock only offered about 2" of growth before the seatpost would have been on maximum, which I thought was a bit too little.
He has been riding my 26ich wheeled Genesis whenever he can and does a fairly good job of it, so I've decided to go for a small adult 26" wheeled bike instead. I found the Cube Kid260 online for about £300, so that's what's waiting for him come his birthday.
Unfortunately I can't comment on how it fits him currently as his birthday is next week! The Cube website says that it's for kids who are from 140cm tall, which he just about fits (although he's not especially tall for his age).
Hope this helps,
Ben
(Edit: of course you may really need a 24" wheel, so of those that I've seen I thought the Speccy Hotrock and the Gary Fisher Precalibre were best. Also, the On-One site has a case study of a lad riding a 14" Inbred with 26" wheels….)