This achievement is unlocked for finishing the game with the a child in your group.
The state of the child (Negative mood, injuries etc ...) do not affect this achievement as long as they are present on the final turn.
Note: I am unsure whether the child has to be an original character that you started the game with. If that could be confirmed in the comments.
Edit: Confirmed below from MOFOwhosinchrge. The child does not need to be an original starting character for this achievement to unlock.