Hi Patty! EVERY child is assured a place in the children's programming, whenever they are open. They only accept cruise reservations for a certain number of children and after that point accept no more children, so they can assure every child a space in the programming.
If you have kids under three, however, and are using Flounder's Reef, you DO need to make reservations for that on embarkation day and it is possible they would not be available on a certain date if they were full.