I have a household where the son brought home his new girlfriend (she was part of another of my households that I wanted to link together by marriage). Upon meeting, the dad had romantic and flirty feelings towards the new girlfriend which was not what I wanted at all and wasn't really making a whole lot of sense. He isn't a romantic sim by design and there was nothing in the room or conversations taking place that would have created a romantic mood. Turns out, one of the active Neighborhood Action Plans was "Free Love" and that was creating the overly amorous feelings. Perhaps that's what's happening here? Once I changed that, the sims went back to normal.
First time I ever used a cheat to adjust the settings in my game was to correct that. I went through every neighborhood in every world and adjusted that and a few other troublesome ones out of the list as I find some make the game absolutely unplayable. I hate the Neighborhood Action Plans!!
As for children developing crushes on people - that does happen. But it lacks certain components that crushes at older ages have. It makes them more on the innocent side than anything else and the child grows out of them. An adult may find them cute or endearing but will not want to act on them or take them further. In the Sims, "love" lacks much of the nuances it has in reality as it all seems to be related to eros (romance related) not philia (friendship related). So even a generic hug will feed into the romance meter. At least - that's my opinion - I could be wrong.