We traveled last time with 4 generations and will be doing so again, though my grandfather isn't with us anymore. Having one central planner is essential. I consult everyone about restaurants and rides and then fit it all together with park hours, crowd calendars, and touring plans. My parents and grandma are very easy going about it and very helpful with the kids. Last time my youngest was 2 and my mom took him back for a nap a couple times while we stayed in the park with the oldest. A very important thing is to have a plan. That keeps everyone going in the same direction. Be willing to be flexible though. If it's raining and you are miserable, go back and get dry even if you haven't finished. Be willing to go separate ways if needed. My oldest had a meltdown one morning so the best way to deal with it was for hubby to stay and calm him down and the rest of us to get out of the way. We were headed to Epcot and realized it didn't make much sense for the 2 yo and went to MK instead. After the meltdown was resolved they continued on to Epcot where we met back up at lunch. My grandparents didn't go with us to
MNSSHP opting for an early evening at the resort. This time my inlaws will also be joining us for a few days so flexibility will be key with 9 people. If you have several people who want to be the chief, run the other way or tour separately with a few shared meals. We did USO a little over a year ago with my parents, grandma, and brother's family. SIL is her way or the highway with little regard for our children in that equation, only hers, and that trip was not pleasant. My mom spent a good deal of time in the hotel with SIL and the baby who had to follow a rigid napping and eating schedule and missed out on a lot with the rest of us. It seemed like we stopped every 5 minutes and in general spent a lot of time doing nothing because she made it clear that she would not follow a touring plan, least of all if I made it!