Ahoy, Ben!
What a great trip to take your adult children on! I am looking forward to being able to do a similar trip when my girls are both over 18 (is it REALLY only 6 years away???).
You are correct that the drinking age on a Disney Cruise Line ship is 21. However, the age to be in the "adult district" at any time (meaning evenings) is 18. Your daughter is welcome to hang out and have non-alcoholic drinks to her heart's content. In fact, there is a group onboard that caters to the 18-20 crowd. They often hang out in the clubs together, enjoying the evening entertainment.
Be sure to book Palo or Remy for a meal. My favorite meal on any Disney Cruise Line vacation is brunch at
Palo. It is not only delicious, but a milestone, since it is an adult-exclusive restaurant. Also, when visiting Castaway Cay, head to Serenity Bay to enjoy a kid-free beach and BBQ!
I am so glad you found the Disney Parks Moms Panel to ask your cruise question. Please visit us again if you have anything else arise before you set sail!
Happy Cruising, Tracy V.