OP, its possible you're referring to some of the "ticketed" events you may have read about. The "Princess Tea" is an event that I believe you can book ahead of schedule. However, the princess meet and greets, the Disney Junior breakfast, and the gingerbread house making (if you're doing a holiday cruise), are all things you register for when you arrive at the ship.
You could also be talking about "registering" for the kids club in general, which can be done in advance.