If you don't mind teaching with a semi auto Magnum Research makes a dandy little 22 with an ajustable stock. It will fit the smallest frame child and adjust to fit them when their full grown. It's a bit pricey but they're a lot of fun to shoot and pretty darn accurate too. The stock is made by Blackhawk. You can also buy a Blackhawk stocks to fit Remington actions and a few others. An ajustable stock is the best solution for any youth and will fit them for a long time.
http://www.magnumresearch.com/Expand...ctCode=MLR22TB