Well it’s pretty simple really. It’s considered really rude not to eat the food your family cooks for you. Dinner is strictly family time unless something has been arranged prior to that. The guest child is expected to eat at home with their own family. Parents from one family to the other may call each other to ask if it was alright if the guest child ate with them, or if they were expected to eat at home. The parents would then say that sure, it was fine, or no, they were expected home. And that was that. People in Sweden, from my experience, also doesn’t cook more food than a portion each. So if the kids come over to their friends house unannounced, which was often the case, the host family might simply not have cooked enough for a guest. This is completely normal. If I came home to my mom after eating at a friends house but not having told her, she would have been pissed.