You will NEVER be able to keep the whole group together for more than a meal and maybe one or two attractions at a time, so don't even try.
The best course of action will be to send out a poll of major attractions for everybody to rate how much they want to experience them, rated on a scale of 1-5 (for the youngest kids, Mom and Dad might have to fill it out for them). Once you have the polls, you can match up 2 kids and an adult or two to form smaller touring groups for each park, and map out a rough touring plan for them. The groups would mix and match each say, as different people will want to do different attractions in each park, so the kids may or may not be with their actual parents on any given day.
Print out the touring plans for each group for each day, including meal times and meeting places for larger group events like parades and fireworks, and cut them down to pocket-size pieces of paper (about 3x5 should do it), and laminate a few copies for each group to carry with them.
Each adult should have a cell phone, and you should pre-program everyones phones with everyone elses numbers. You can also print out a master plan with everybody's phone numbers, plus 407-WDW-DINE, your resort's phone number, and any other general info about the group's plans.
This will mean a lot of work for you, but planning is half the fun anyway. And planning ahead will mean fewer melt-downs in the parks - "But I don't WANNA see Mickey's Philharmagic, I wanna ride Pirates again!" - "I don't wanna stand here all day watching this dumb parade, I wanna ride some rides!" - "I've been stuck on the kiddie rides all day, I wanna get on some roller coasters!"
Get the right people to the right attractions and everybody will have a better time.