We used the nursery a couple of times on our cruise in January. I don't know about the 3 and 4 night cruises, but on the 7 night cruises, they have times set aside that you can take your child into Flounder's to play while you supervise them. This was good because it allowed our son to be familiar with the facility before dropped him off to stay. We'll use Flounder's again when we cruise in January!
You also may not know this, but you can take your child into the Oceaneer Club anytime to play, as long as you are with the child to supervise them. Trevor loved climbing in the treehouse and enjoyed all the toys. The staff was great to him - the included him in story time and activities, we just had to be there with him.