You're assuming the children are evenly distributed and buildings are attacked randomly and evenly. None of which is true.
Many buildings have been demolished with explosives or bulldozers while empty. Many buildings were damaged during combat with walls being breached to provide access.
In many cases buildings were attacked in areas that have been ordered to evacuate. (yes not all cases).
There's no doubt many children have been killed. We've all seen terrible photos of individual children dead. I don't think it's "tens of thousands" and I'm not sure we'll have accurate numbers. Other than that as you note there's a large number of children and so they've definitely and unfortunately impacted.