I think it's the media stirring things up. For some reason, they just love assigning "starring roles" to children and declaring that the kids "stole the show" at weddings, jubilees, funerals, and now coronations.

I doubt Archie has been invited to the Abbey. Nobody has a clue whether he's well-behaved or not, and his internal clock will be screwed up from the 8-hour time change. His parents aren't going to arrive days before the event to allow enough time for their kids to overcome jet lag. And Charles isn't going to want to share the spotlight with the birthday boy.