I just cruised with my 17-month old twins for the first time on the Fantasy. It was only a 3-night so we were only able to book 10 hours for each of them online. I think it is more for longer cruises, but not sure. You can book more onboard, and/or just drop in and see if they have room, which we did.
Online - I booked times that I knew we would definitely need to have them dropped off. A few hours for an excursion on CC, and every night from 9:30pm until 11pm. We had some hours left over, so I did a few hours on a sea day during their nap times so the adults could enjoy the adult areas of the ship.
The key is to use the nursery for nap and bed times. We had every meal with the babies, but they do feed them meals in the nursery if you wish. We tried to watch the show the first night with the babies, but we soon realized they were not going to stay still for the next hour or so, so we quickly ran over to the nursery and they were able to take them in at 8:30pm for the hour before our reserved time. We pretty much did this every night, and had a great time watching the stage shows and enjoying the adult entertainment.
The nursery will follow all your directions for your children. We asked them to give them milk and water before bed, and change them into their jammies, and what time we wanted them in bed. They did wake up when we picked them up at night, but pretty easily went back to sleep upon returning to the room.
During our excursion the morning on CC, they napped and were changed into their bathing suits so we could pick them up and go to the beach and eat lunch on the island.
I understand all babies are different, and maybe we were lucky with our twins being so good on our “trial” cruise run, but the nursery was amazing for us. If your kids are really upset, they will text or call you. We were only messaged once, but not a big deal. You can stop by and check on them, or call them on any phone on the ship. The one thing I didn’t want to do was sit in our room while our babies were napping, so you can use the nursery to suit your own needs.