My daughter was 3 1/2 when we cruised. She had never been in daycare, and other than 2 times when a close family friend watched her she had always been with either Mom, Dad or Grandma. I was a bit worried she wouldn't want to stay. We dropped her off the first night and she had a lot of fun. She hardly ever wanted to leave. One day we dropped her off around 9 am for som activity and we couldn't get her to leave until 11 pm that night. We stopped by every few hours asking her if she wanted to swim, or do such and such, but the answer was always no. She just wanted to stay and play with her friends. All the other nights we had her eat with us, but that night I decided if she wanted to say for dinner, that was fine.
We did get paged a couple of times to come pick her up and when we got there and started to leave she told us she wanted to go somewhere else. The problem was she couldn't figure out how to get to the area with the computers, games and dress up clothes. Either she didn't ask or the people she asked didn't understand her, because when we took her in there she decided she wanted to stay longer and play. We played with her in there a bit, then left her for another couple hours to come back and get her.
She doesn't know it yet, but we're on the 14 day repositioning cruise, and I'm sure she'll have as much fun then as she did last time. She still constantly talks about the Mickey boat and wants to know when we're going back.