Surprised you didn't include Alfred Enoch, he was great in How to Get Away with Murder. Emma Thompson has had notable roles, but I still feel she's not that great an actor.

Helena Bonham Carter (Bellatrix)

Emma Thompson and Helena Bonham Carter were both already hugely successful before Harry Potter. I assume that that is why the poll is specifically about the kids, whose careers really began with Harry Potter.

^^^ that was supposed to be Emma Watson, not Thompson. I don't know how that happened.