It is intentional but being able to move in a child sim without his/her parents is just wrong on all levels. I know this is a game, but I always compare it to real life ... just seems like kidnapping the poor child. You befriend the parent, ask him to move in, then say: "hey sorry bud, I don't want you to move in, I just want your child. Off you go now."
@oshizu I apologized if I mess up your dynasty. I was just so used to sims 3 not being able to move in children by themselves, and didn't realize sims 4 you can do it.