How do we know what the characters look like? We've only got a few abstract drawings of Harry and Ron, and only vague descriptions of the others. They'd have to go by the actors.

Plus, they'd have to choose the best lookalikes from among the entrants. If I were the only entrant, I'd win, even though I look about as much like any of the actors (or characters) as I do like a fish.

IMO, these kids look enough like the actors to be distantly related to them, but not exact doubles. Even if Megan is wearing a wig, lots of times people who enter lookalike contests do stuff like that to look more like the person. She's a very pretty girl regardless.