Kimberly,
Sounds like you are gearing up for a night out in Walt Disney World sans kids! While traveling to this magical place with your children is lots of fun, it is also nice to have an evening to yourselves. Anyway, to answer your question, reservations are recommended but are not required. Walk-ins are welcome and taken at a first come, first served basis but based on availability. These reservations can be made up to 180 days in advance. Since space is limited, I would book it as soon as you are able to guarantee the date you are hoping for.
The Sandcastle Club is a nautical-themed activity center and the kids will have a ball making crafts, playing video games and interacting with other kids their ages. Dinner and snacks are also provided. Dinner usually consists of classic kids' favorites such as chicken fingers, macaroni and cheese and PB&J sandwiches. Children must be potty trained and it is open to kids ages 3-12. Sandcastle Club is located just off the main lobby of the Beach Club near the gift shop. The last I heard, the cost per child was $11.50/hour with a 2-hour minimum. However, these rates could change without notice, so be sure to inquire about that when making your reservation. You can call (407) WDW-DINE (939-3463) for reservations. It opens at 4:30pm and closes at midnight. So, make sure you are back to pick them up before you turn into a pumpkin!
I hope you enjoy your night out and if you need any suggestions on how to spend an adults-only evening in Walt Disney World, please feel free to stop back by and ask me my recommendations!
~Allison L