That's one reason (the main reason) why Mrs U and I have never taken a Disney Cruise. Too many kids running all over the ship and this would likely add even more.
Great deal for a family though---
They are only free if you can stow them in your overhead compartment.
Expect to pay big bucks if they don't fit and you have to check them.
Better to use Southwest Cruise lines. They allow each person to check two (!) kids for free.
They also give you a half can of soda and a snack for free. Expect to pay an extra $5 for this service on Disney Cruise Lines.