Your group and kids' ages are a lot like what I had on my Oct '14 trip.
I had the kids aged 6, 9, 13 and my niece and nephew were 3 and 4. I highly recommend that the parents of the youngest keep nap needs in mind and don't insist on doing everything the rest of the group does (if they continue touring through nap time). I like to choose which park on which day based on the crowd calendars on easyWDW or touringplans.com
For a mixed age group, I'm going to assume that at least some part of the group wants to ride every ride/attraction, and take turns sitting with little guys for the things they can't do. I like planning on getting to MK at opening - lines are shortest in the first 90 min- 2 hours. Schedule Fastpasses for things the whole group wants to do together to start around 10:30 or 11. Do FP, lunch, FP FP and then people can divide up a little for rest, thrill rides, different interest, etc (my group actually stayed mostly together, but we didn't feel like we HAD to). We met again for a sit-down dinner most nights. A character meal is fun, especially for groups, and lets you meet some characters without standing in line during the day.
For MK, I'd focus mostly on one side of the park (Adventureland/Frontierland/ liberty square) one one day, then the other side (tomorrowland & Fantasyland) on the other day. Slight adjustments depending on when you can get your chosen fastpasses.
For DHS, you definitely need to peruse a park schedule, because it has lots of shows, so times are pre-set vs rides where you just show up as you like.
The Fastpass+ attractions and priorities thread is amazingly helpful once you have an idea what attractions you're interested in, it will help you choose what to pick Fastpasses for.
can you add a day and visit animal kingdom?
