I will second the Ping recommendation. Good quality, online fitting tool. Like trevor said, you can send them in once time for free and they will reshaft to a longer length as the child grows. By the time the kid outgrows the reshaft you'll be ready for their first adult set. I did this with my son as well a few years ago. I think I was able to sell the Ping set for $500 after he was done with them, too.