I know many people don't agree with our approach, but here goes!
My kids are now 24, 16 and 11, so we have always had a huge age gap. When they were younger, we would always split up in the morning, meet at lunch time and then spend the rest of the day together.
I would usually take the older kids and do the wilder things like space mountain or RnR, while DH would take the younger kids and do the tamer rides.
We would then do things together as a family the rest of the day and evening.
There have also been times that we have even split-up and gone to different parks and then meet later in the day. When the younger kids were really young, they were worn out by mid-week and were ready for a down day, while on the other hand the oldest was still wanting to go,go,go. There have been a couple of times I have taken the oldest one to Universal or IoA and DH stayed at the hotel and had a pool day.
I know many people would never split up, but it works for us to split up for part of the day and then spend the rest of the time together.
Also, with the large age difference, we have always done more one-n-one with the kids than we would of if they were closer in age, so it never seemed strange to the kids that one kid was doing something they weren't. I didn't want my older kids to miss movies and activities that were age appropriate for them just because their younger sibling wasn't old enough to go. I know many people don't agree with the way we do things. They think its important for a family vacation, that the family needs to stay together, but I would rather have a little one on one time with the kids than to have wiggly kids waiting in a line for a ride they won't be riding.