My nine-year old rides a REDLINE conquest - 24 inch wheels. Throw some slicks on and you have a summer bike. Great for confidence on and off the road, my only issue (like with most kids bikes) is the compromise in component selection for cost over weight. I bought used, upgraded, bars/stem, BB, Cassette (to bring weight down)and brakes (for handling/safety). We put SPD pedals on this year - still getting used to them.
He has a mountain bike for riding with friends and bombing around the neighbourhood. (the benefit of an older brother and hand-me downs). If only one bike, MTB may be best all-round solution.
In about two years, he'll be ready for a full-sized bike and not have any other transition issues to deal with, vs coming from riding only MTB.
IMO, a cross bike is best for juniors for both CX and road until they are 15+, where the weight and geometry differences might actually matter for enjoyment/performance.
My $0.02 - hope it helps.
My kid uses Novara Pulse. It has 26 inch wheels, works great on the road and cross. Seems durable also.
My son spent two years on the Redline - very adjustable with a long seatpost and flip/flop stem.
He loved it. It meant going to races with his dad was way more fun.
We replaced it wih a Giant TCX W (from the forums here).
It has a real bike parts, is light weight and fits like it was custom.
BTW he's 12 now.......
My two younger kids are on Redline Conquest sz 44. Looked at getting the Conquest 24, but for the money, the 44 was a better investment.
The older of the two first got on about 3 years ago, when he was 10, and at 13 and two growth spurts later (and 5' 4") he still has room left on the 44. The only thing that's changed is seatpost and stem length. My younger just turned 10 and she's on the same size frame, but with a slammed post and uber-short stem.
The only other thing that you have to change for kids is bars--the super deep bars are, well, too deep. For my son we found a womens bar designed for ladies with small hands and that's worked really well. For my daughter, a friend in town had a set of bars left from his son's now-busted 24" wheel road frame, and we've used those.
The only thing I would change on these if I could would be the build kit. They both use triples, and for the kids that's overkill and just leads to dropped chains even with a chainkeeper. One of the bikes has TIagra (9s) while the other has Sora (8s). Neither is particularly good for small hands. I'm told that SRAM works better for small hands, but I have no direct experience.
These both use 24" 520 ISO wheels (not standard kids MTB 24" wheels)
Kona Jake 2-4; http://2k12.konaworld.com/kids.cfm?content=jake_2-4
Redline Conquest 24"; http://www.redlinebicycles.com/bikes/cyclocross/2013-conquest-24
BTW, if anyone has a front fork for either of these 2 bike, I am in the market.