Metal Gear Solid V. The Phantom Pain (2015) is praised for its ability to adress mature themes in its narrative. Developer and living legend Hideo Kojima expressed explicitly his whish to do so. And he succeeded: I have found an almost bare naked woman worrior (Quite), bombs hidden in a vigina, child soldiers, forced labor, mutilation, love, revenge, betrayel, and so on. But one taboo seems to have been too great and strong to break, even for Kojima. You cannot kill children.
Yes, there are children in the game. And yes, they behave like adults in shooting each other and the player alike. But no, you cannot kill them. Everytime you kill a child soldier, the mission ends automatically in a failure. Adult enemies can be dealt with in several ways: killing or tranquilizing them. Children cannot be killed. It is the last taboo of the adult gaming industry.