...or upset about circumstances that may seem too hard or unfair to them. I think they are much more likely to "get over it" if you sit with them, let them cry/complain/whatever for a few minutes, help them get their emotions after being upset back under control, and then talk them through it.

I know a lot of parents who think their kids "got over it" because they didn't/don't talk about it any more. Usually that's just evidence that they aren't talking to their parents about it anymore, because they know their parents don't care. But it isn't evidence that they are actually "over it".