Welcome aboard, Meredith!
How awesome to hear that you'll be sailing on your very first Disney Cruise Line voyage next year. Your whole family will have such a spectacular time - I'm sure of it! Your kids will enjoy themselves at the Youth Clubs and you... yes, YOU... can have a real vacation too! It's one of the best things about enjoying your Disney Cruise -- there really is something for everyone in the family.
If your daughter turns 3 while on your cruise and she is already fully potty trained, then she will absolutely be able to join in on the fun at the
Oceaneer Club. Sounds like your son is also in that same age group (3-12 years old) -- which means, you'll just have to deal with them wanting to both stay at the Oceaneer Club all day and all night :-). They will have loads of fun in there, and the Youth Counselors are pretty cool to hang around with - they really do such a great job with the kids. While the kids are in the Oceaneer Club, Parents won't have to think too hard about what to do on their own. There's lots of big kid activities that you can do around the ship too.
On embarkation day, I would suggest stopping by the Oceaneer Club and playing in there with your kids, during Open House. Since your son will be in there with your daughter - she might not have any problems being left. But it's always nice to see where they'll be playing at, since they'll be spending a good amount of time in there. The themed spaces are pretty cool - you'll want to see the different activities that they'll be enjoying and you can also chat with the counselors to find out more about age appropriate activities that they have in store for the kids.
Hope this helps, Meredith. If you have any additional questions, please do drop us another note. We're always happy to help!
Have a great day!
~ Dolly