Feel free to add answers that are broadly about the series as a whole.
Inspired by the market about season 1: https://manifold.markets/jim/what-will-be-true-of-the-new-harry#RSO5Rz2pStdz. If you have season 1 thoughts, check over there.
Closing date will be extended in the future if necessary.
@BrunoParga Harry, Hermione, the five youngest Weasleys, Draco, Neville, Dudley, Luna, Cedric. They are the 12 most often mentioned children: https://gist.github.com/jennynz/94819293db37952b25423d6af813b125
My other comments aren't loading. Let me amend them: the actor must be on screen in three episodes, and the character must be named. Their name appearing only in the credits, or in the books, doesn't count; the character's name must be said or written in an actual scene of at least three episodes of the show.
Multiple names referring to the same person count; so if the main antagonist was only referred to once as Voldemort, once as Tom Riddle and once as You-Know-Who, that would still count.
@BrunoParga besides the three episodes stipulation of the other answer, this must be an actor under 18 years old when filming at least one of the episodes they're in.
@BrunoParga must play a named character appearing in at least three episodes (not necessarily in the same person, but flashbacks showing the exact same scene don't count separately).