Hi, Amina!
Congratulations on your first family cruise aboard a Disney Cruise Line ship! With Disney making it so easy to relax for moms, you may just wonder why you haven't sailed before now!
Scuttle's Cove is a great place for supervised as well as free play for kids on Castaway Cay. The Youth Activities Counselors from Disney's Oceaneer Club and Lab staff this area and can securely check kids in and out using their Mickey Bands and a lap-top. So, you can plan for your 5-year-old to enjoy this area while you enjoy some adult time on the island. They'll even feed the kids lunch from Cookie's!
Whether or not there are nursery services available on the island really depends on several factors including the age and mobility of your child and even the weather. Obviously, there aren't as many "comforting" resources for the little ones on the island as there are in a nursery setting back on the ship. Therefore, you'll need to reserve your nursery time during your day at Castaway Cay, and then once you're on board, you can make your request for Scuttle's Cove.
I hope that helps you plan. Let me know if you think of other questions. I'm here for you!
Have a wonderful day and a magical vacation. Bon voyage!
Andi