Welcome aboard, Nancy! Your whole family must be looking forward to setting sail for the very first time on a Disney cruise. Well, I can assure you that you've chosen the right cruise line with which to sail, as Disney Cruise Line offers something for everyone during your vacation!
It sounds like you've been browsing all of the exciting
Port Adventures offered by Disney Cruise Line. Aren't there some fabulous ones? Your Port Adventure cost will be charged to your stateroom account, so no need to have cash for that payment. Most of the tourist areas in the ports accept US Dollars and major credit cards, so there shouldn't be a real reason to transfer too many Dollars to Pesos for your quick port stops in Mexico. In fact, if you book your adventure through Disney, pretty much everything is included. The description of the adventure will note if lunch is included or not. If not, plan to bring some cash or a credit card to purchase food or extras.
My family has sailed to many ports in the Caribbean and Bahamas with Disney Cruise Line and we've never had to change any US Dollars to the local currency, as most of the places we visit are happy to accept our US Dollars. I hope that helps as you continue to plan your cruise, Nancy. Please let me know if you have other questions before you set sail!