On the battlefield, the dead don't get up and walk away when the movie ends. War is not a game. Children are not pawns on a chessboard or puppets on a string. Children, by nature, are easily wounded - easily and severely, in body and in mind. Even if soldiers only aim for the legs, we have to remember that the legs of adults are often at eye-level for children.
Children are physically weaker and are easily injured.
Wounds that would not be fatal for an adult can be deadly for a child. Children are more likely to take risks. They are more likely to be less cautious and thus expose themselves to far greater danger. Consequently, they are far more likely to get hurt. Tragically, the facts prove this to be the case.