That would be so awesome! AND... Since Bethesda will not allow children to be killed, YOU'RE IMMORTAL!
Unfortunately, I do not know of a way to make that happen...
Well considering I'm bordering on being a senior citizen in real life, being able to play as a teenager is awesome enough for me. Of course, I always tend to cringe when someone in the game calls me "kid".
There are a couple of mods that allow you to play as a child
http://www.nexusmods.com/fallout3/mods/1669/?
Then this one is used to make the voices work right
http://www.nexusmods.com/fallout3/mods/16305/?