Yes and no.
We usually do trips between 8 and 10 days. About three years ago we went to Disney for a friend's wedding. We arrived on a Thursday morning and stayed until the following Monday afternoon. We had the wedding on Saturday, so that day didn't really "count."
When we arrived on that Thursday, we drank and ate around the world with the bride and groom. That night my family dined at Brown Derby and spent the night at Hollywood Studios. On Friday, the boys went golfing for the day while the girls went to Magic Kingdom. I can't remember specifically what we did that night. I'd assume we laid low, as the wedding was the next day.
Saturday was the wedding - so we didn't go to the parks.
On Sunday we went to Hollywood Studios for the day and then relaxed at the resort. That night we did a group dinner at O'Hana and went to MK for late-night EMHs that night. Monday we relaxed at the pool before heading back to NY.
The trip was short and certainly unconventional. We didn't accomplish everything we wanted - time didn't allow it - but it wound up being one of the most memorable trips to date. I think it was having a huge group of people that made it so special. We used the EMH to our advantage - staying very late on Sunday night (almost 2am!)
Obviously your trip with young children will be different. You said you're going in the fall - definitely check the schedules and make sure your time at MK doesn't fall during
MNSSHP or
MVMCP. This is a
separately ticketed event so it could really mess up your plans.
I'd recommend arriving early in the morning and tacking some rides. Fortunately, fall is much less crowded than the rest of the year (depending on when you go.) Around noon or 1pm, eat some lunch and go back to the resort for a nap and to relax. You can do dinner and another park or return to the park after dinner for an hour or two.
Make a list of your must-do attractions. See Wishes, Illuminations and Fantasmic. List the rides you'd like to go on and work from there. Next, schedule your FP+ to meet your schedule. Don't worry about "maximizing" the use. On an 8-day trip, maximizing the value of your FP (i.e. getting it for the afternoon or doing a ride just because it's hard to get on) makes sense. During a shortened trip, just hit the rides
you want to do.
Hit the classics. In Magic Kingdom, go on Dumbo and Peter Pan and Small World etc. See the characters and the shows. Right now, the novelty of Fantasyland is still pretty strong. It would be great to get a FP+ for 7DMT but if you can't, don't wait 2 hours in line to go on. It's a great ride, but it's not worth the 2 hours. Every hour of a shortened trip is vital. You can easily get on at least two rides in that time! The novelty will wear off by the next time you go there.
I love Downtown Disney and Disney Springs, but skip it this trip. You mentioned that you have family in Orlando. Even if you can't afford a Disney vacation or don't feel like going doesn't mean you can't go to Downtown Disney when visiting Orlando. Save it for a longer trip or swing by when visiting the family.
Don't expect to accomplish everything. Set realistic expectations with your kids and most of all, have fun!