That young template would make for some pretty terrifying swordsages. Base race whisper gnome, grab shadow blade and weapon finesse and dual wield daggers. Grab Faerie Mysteries Initiate too to take care of hitpoints and provide a dose of heavily implied creepiness.

Anyway, kids are more flexible than adults, not necessarily more dexterous. I recommend just setting size to small and not applying any stat modifiers, just put your stats the way you think a kids stats should look and don't worry about it too much.