Family of 5 Vacation

Family of 5 Vacation

Hello,
So i have a family of 5 inquiring about Planet Hollywood in Cancun. 2 adults, 3 kids 11, 7 and 1. When I put 5 people on the dates they requested, it says its sold out. When i put 4 people it is not sold out. My question is, how can we book the family then add the baby? The mom is asking for her to be on the lap. Do we need a plane ticket for the baby? Then the resort, do we call and add the baby? Or they just show up there with the baby?