It's not killing children for crimes they might commit. It's killing children due to them being part of a larger system that has to be eradicated. In this sense the children are as much victims of the system as anyone else is.
The circumstances that allowed china to turn Puyi into a janitor were not the same circumstances that forced the hand of the soviets to pull the trigger without trial. Had the royal family fallen into the White Army's embrace, then the USSR would never truly be safe from a monarchist reaction.