I saw this the other day. It might be a little small, but it looks pretty sweet:
Tons of options at 48/50, not really a kids size. Cannondale and others are making 44's and 46's. You should be able to easily pick up a used bike in those sizes for sub 1k.
Much harder trying to find something in-between the 24" wheel CX bikes and the 44's. Novara from REI is right in the middle and Surly makes a steel 42 but both are tanks, kids have a hard time lifting them over the barriers. I'm looking for bikes for the 10-12 year olds and not a lot of good options.
Independent Fabrications makes a Ti 42 but not many parents are willing to drop 3-4k for a frameset for a 11 year old...