Hi, Stephanie!
If anyone understands varied ages all under the same roof, it's me -- I have 16, 14, 11 and 7, and we've been traveling together since the first one was six months old. As each of them have come along, finding activities on vacation to do together seems to be one of the greatest challenges.
Disney Cruise Line makes it easy for kids or adults to spend time on the ships individually, as a family or with only one or two other family members. While the adults in your group are enjoying dining, spa experiences, Port Adventures, adult activities/events or the bars and clubs -- the kids can simultaneously spend time in the nursery (4 month old), Disney's Oceaneer Club/Lab (6 year old) and Edge (11 year old). You'll simply need to make prior reservations for the nursery -- the other kids' clubs are come and go.
If you happen to notice an activity or event at Disney's Oceaneer Club/Lab which your 11 year old may enjoy with his/her sibling, you can ask the counselors if he/she can join the younger children. However, the nursery is limited to just children who have reservations -- since babies and toddlers aren't as tolerant of people coming and going in that environment.
There is much fun to be had as a family with deck parties, live shows, rotational dining, movies, game shows, karaoke, character greetings and much, much more! See
Family Cruise Activities on DisneyCruise.com for the seemingly endless possibilities.
You've made an excellent choice for your family's vacation. Please let me know if I can provide more insight for you. Have a wonderful day and a magical vacation. Bon voyage!
Andi