As others have posted, if your DD is fully potty trained (no assistance needed for anything) than she will be able to use the Oceaneer's Club. Their hours are 9 am to midnight, I believe. If your DD is not potty trained, then Flounders is available. The hours at Flounders vary somewhat. For us (7 night Mex Riviera), the hours were noon to 4 and 6 to midnight the first sea day, then 10-2 and 6-midnight on Monday and Friday (our other sea days). On the days in port, the hours were 9 to 1 (I think) and 6 - midnight.
One thing to remember about Flounders is they do not feed the children, so your child needs to be fed before arriving and picked up for her next meal. At the Club, they take the kids to one of the restaurants for lunch and dinner.
Do a search on this board. Others have posted about how they have handled young ones and "potty trained-ness" at the club.
Good Luck and have a wonderful cruise.