All passengers in the 3-12 year old age group are automatically eligible for the kids' programming, so you don't need to register when you book online activities the way you do for the Nursery.
Pre-COVID, there was a page during Online Check-in for families with kids 3-12 at which you could indicate who was authorized to pick up the child and provide the password to be used for check-out. My kids are now too old for the Club/Lab, so I don't know if that is still part of Online Check-in.