Hi Rosemary,
Welcome to the Moms Panel! Yes! Your children can be pre-registered in the clubs ahead of time, but it isn't required to save a spot. The reservations process tracks the number of guests/child ratios to ensure the clubs will not be over capacity during your voyage.
For instance if you are trying to book a cruise, it may be sold out if you have children on your reservation - yet if two adults tried to book the same cruise they would find availability. This is how the club occupancy levels are maintained.
Now I have to mention that I did pre-register all of my children online for our last cruise - but once I arrived I had to fill out another form that asked the same information. I found it redundant, and didn't quite understand what benefit pre-registering was.
So sure, if you have the time - go for it. I will be an option under "My Cruise Plans" when you are on the
www.disneycruise.com website. You will need to register/log-in to see this feature.
Have a great time!