Avenger: Infinity War will be known, at least until the next big Marvel thing comes, as "the movie where they killed everyone," and there's no way around that. It has become a major joke of pop culture that Thanos will snap and various people dissolve into dust. Throw a stone and you will find a meme based on it. It's a defining element of this movie, and the moment the fans saw it, the complaints started.
They know the ones: "They will undo these anyway, so it does not even count." "There are no stakes in this film if there are magic stones that can reverse everything" and so on.
For the most part, children do not have this concern, not only because they do not necessarily think about retakes and movie marketing (of course, Spider-Man will return because it's profitable), but because at the moment, the story is just as invested as the characters. Iron Man does not know that one day his friends may live again. In the reality in which this story is experienced, the young viewers do not know either, and that has something very pure.