Child

Children are the young inhabitants of the wastes.

Differences between the games
Children may be killed in the first two Fallout games. Killing them gives the player character the Childkiller reputation. Children in Fallout 3, Fallout: New Vegas and Fallout 4 are marked as essential, however, they are also coded so that they cannot be hurt by any kind of attack. They will react to the player character or another NPC assaulting them, always fleeing instead of retaliating against their assailant. The only child that can die in Fallout 3 is the Lone Wanderer in Tranquility Lane, a virtual reality simulation.

Considerations
Bethesda faced a similar problem when making The Elder Scrolls IV: Oblivion. They considered adding children to make the game more realistic, but this was a controversial choice: if the children could be killed, Bethesda likely would have been subject to serious controversy and legal problems that would arise from making it possible to murder juveniles in the game. Bethesda's response was to simply make the characters invincible. This once again raised issues, because the tactic would undermine the purpose of adding children in the first place, namely, realism. Eventually, Bethesda scrapped the idea for The Elder Scrolls IV: Oblivion altogether, but in Fallout 3 they went the other way, allowing children for authenticity, but making them invincible. This is the same in Fallout: New Vegas. In Fallout 4, children remain invincible, but they, like in previous games, will react to being assaulted. They will always cower and flee instead of retaliating against their attacker.

Fallout 3

 * Harden Simms and Maggie, two child-inhabitants of Megaton, disappear from the game if the town is blown up by detonating the atomic bomb. They presumably die in the blast.
 * Arthur Maxson, the only child-inhabitant of the Citadel, will disappear from the game if the Citadel is blown up in the final quest of Broken Steel.
 * Timmy Neusbaum of Tranquility Lane is removed when the task to slaughter the residents arises, or when the Chinese invasion program starts. A unique garden gnome called "Timmy the Gnome" is placed on the Neusbaum doorstep, indicating that Timmy's Tranquility avatar has been altered. After the Lone Wanderer leaves the simulation (after using the failsafe), examining Timmy's vitals verifies his death.
 * Children cannot be targeted in V.A.T.S.. However, if they are accidentally injured, they can sustain crippled limbs.
 * Since children cannot be harmed, they will generally not fight the player character, even if they are armed. Instead, if attacked, they will flee.
 * The Mister Sandman perk cannot be used on children.

Fallout: New Vegas

 * The kids running around in Freeside who chase the giant rat have the same voice, regardless of gender.
 * Attacking children results in infamy with the faction they are associated with, even though they cannot be hurt.

Fallout 4
Children cannot be targeted in V.A.T.S.

Fallout 76
Children are flagged as "Ghosts" and are not part of the combat system.