my family did a family vacation of 20 back in 2000, i'm also in the process of planning my wdw wedding for 55.
some suggestions:
-stay at the same resort, we did it was nice eating breakfast, swimming at the pool together or splitting into groups, you could even get a group code discount if you book 10+ guest rooms.
-plan some down time, i dont suggest every moment together. plan specific events like a welcome dinner, farewell breakfast, day at the park, afternoon swimming, kids only pirate cruise, hayride, golf & spa, fantasmic dinner package or adults only night at pleasure island - enlist the help of some family babysitters to young to old to join at pleasure island but old enough to keep watch or book in room babysitting, our family used kids nite out for the young ones for an adult only night. we did these events together as a group then had time to enjoy alone & in smaller immediate groups.
-you will all have a great time, but you all must compromise, again suggestion above alone time, everyone will not always want to do the same thing.
-some airlines will offer group discounts, we all flew american airlines & got 10% off each rt fare.
-join the grand gatherings at waltdisneyworld.com for info on large group travel, special events & online polls for your group
last suggestion, have fun!