How do you be a child on Fallout 3?

Answer 1

Play through the game until you get the BB gun. After receiving it, do not shoot the targets. Open the console menu and type "coc MegatonPlaza", without the quotes. This will teleport you outside the Vault, and into Megaton. Bear in mind that this will make the game essentially unplayable, without using the console to set quests as begun. To fix some of these glitches, find and enter Vault 112, beginning the "Tranquility Lane" quest. This method does have some of it's own glitches, however.

Another idea is to type "setPCyoung 1", again without the quotes, into the console menu. However, I haven't yet tested this.