Whether or not you can be spontaneous at WDW partly depends on what you want and what you expect. If you want to get onto every single attraction, especially the top tier ones and you don't want to wait more than a certain amount of time, you may have to accept less spontaneity. If you want to do everything, see everything, "get your money's worth" by doing a whirlwind tour of all parks, you have to accept less spontaneity. We have been several times, so we don't feel that need but I seem to recall that we haven't ever done that even on our first time; we wanted to enjoy being together and just having fun.
We don't have kids so we don't have to worry about accommodating them, but I've given advice to many friends who have children of varying ages and that advice is: plan for spontaneity, plan for nothing, plan for downtime whether it's at the hotel, by the pool, or sitting on the sidelines watching people while indulging in a snack or drink. Enjoy just being there as much as doing stuff. Never mind kids -
I get cranky if I'm not fed and watered occasionally, if I have to keep going when I'm tired or hot or just overstimulated or my feet hurt, or if I can't go see/do something I want instead of what was planned or what DH wants. I don't care how much was spent on this trip,
I'm going to have a meltdown. Fortunately, he feels the same way. We just make sure we're sensitive to each others' situation; no different for kids except that they may have a shorter fuse (or not - you haven't seen me when I'm hot and thirsty or DH when his feet hurt). Ask them in advance to give you 1-2 things that they really really want to do and that you will do your best to ensure they experience that but it may require changing up other plans to make it work, that you're willing to make changes if
they want to do something else and that they should also let you know when they're tired or want a break.
We do "skeleton" planning - so we do get ADR's (we really, really enjoy the restaurants on WDW property so it is important to us). I use a planning site to determine the least crowded day for each park, pick that park as the "main park" for the day, and then schedule the ADR's. When our FP+ window opens, I schedule those for the planned main park. We have park hoppers so we sometimes go to more than one, especially to enjoy EMH during the scheduled hours.
This describes us very well. I
have to do Victoria & Albert's, would be unhappy if I couldn't eat at Jiko's, and I like several other places but I can be flexible on days, times and which ones specifically. We've made spontaneous dining decision changes a day or two before. We do like to do
MVMCP.
Skeleton plans are intended to be a guide which can be thrown out when you feel like it. And we frequently do. Sometimes, if we feel like the transportation to the park we planned to go to is going to be too long to wait for, we'll take whatever bus comes next and replan on the fly. I always know that I'll find something I'm interested in doing or seeing when I get there so I'm not bothered by the changes.
We have an idea which parks we are going to each day and will have fastpasses for a park each day. We have an idea which rides should be ridden first to avoid lines and which evening show we are going to each evening. We have two or three ADRs and an idea of someplace to eat each meal should nothing come and beat us over the head. Every day has a window in the middle, anywhere from 4-6 hours where nothing at all is planned. We may find some fastpasses, we may go for a swim, hell I'm not averse to a vacation nap.
We've had days when we had lots of plans, got hot and went back to the hotel and bobbed about in the pool, or took a nap, stopped in for a drink somewhere. I enjoyed those days as much as the days we went until 1 am at EMH and did attractions we'd not normally do. And the late late nights are not usually planned. We just felt like we could keep going so we did.
I tend to be a sort of compulsive planner and my husband tends to be more of a "ooh that looks fun, ooh that sound tasty... SQUIRREL!" kind of guy so we generally have to strike a balance on vacation.
I get that from DH during the planning stages (usually after everything has been planned). I used to get frustrated, now I see it as a challenge and even a bit more fun. I even know that he will probably do that during the execution stages, blowing up the replanned planning. I've learned to go with the flow, although I never planned every minute of the day or expected to see everything. One of our more leisurely and spontaneous WDW trips was probably one of our best. Admittedly, we didn't see and do everything we had planned but the main point was that we totally enjoyed ourselves. That's the important thing, that's the real "value" of a vacation, the real "money's worth", not how much you saw and did IMO.
