Is this the sort of view you seek?
Blue and Magenta Castle Path April 2009 Flickr by
Scott S. Baxter, on Flickr
I shot this in 2009 very late at night (obviously), with a 10-20 ultra wide. Can't recall the exact settings, but I was sitting low to the ground near the bottom of the ramp/path to the castle. Presumably, you'd want this view in the morning, with your children walking up the ramp away from you. If so, then that aforementioned earliest-possible ADR is by far your best bet. Even with that, I'd suggest getting in early as the cast members at the turnstiles will allow and making your way up to the castle without delay, and then working as quickly as you comfortably can. There won't be many others in the park at that hour, but many of those who are there will be drawn to the castle.
Another alternative would be to get your children ahead of you on Main Street moving toward the castle -- you could shoot
that view with a standard kit lens pretty easily. If you would visit the Magic Kingdom on a different day before your ADR, you could do a "test zoom" with your camera to determine where you'd need to be on the street to get the view you want, and just make a mental note of where on the street you'd get that view. Then you could come back later and know where you'd want to be. If you find the right spot from which to shoot for the lens you have, you might even have your children walk around the
Partners statue and shoot as they are moving away from you -- that would give you more than one chance to get the shot while still keeping their movements as natural as possible.
Just some thoughts. YMMV.
Scott