We went last year in March with my DH and his parents, and our 2 kids (2 & 5) at the time. And in February, I am going again with my dad, his wife, and our 2 kids (now 3 & 6). Although spring break is busy, and the crowds were pretty nuts, we managed by doing the following:
Trying to get there as early as possible to get to all the slow-loading FL rides (see the sticky Hydroguy thread, it's awesome for planning rides to maximize your time).
Picking one or two "must dos" for each day and then being willing to go with the flow in case something crazy happened (like the day the Alice ride broke down with us in the belly of the ride)
Having one special meeting place per day where everyone knew what it was (not grammatically correct, but I think you can get the jist) in case when we seperated we could find each other
Making sure that we had enough light snacks in my bag and saying to heck with it and buying all the $4 waters we needed to keep the grumpies (from all parties, not just the kids) at bay
Going back to the room for swimming/napping, depending on what was needed each day during the peak times (2-4ish)
Splitting up when needed so our older child could do some slightly more adventurous rides
Making sure that everyone got some "down time"---my FIL & I love the thrill rides, so we snuck off twice for those, MIL & FIL had a romantic evening out one night, and another night they took the kids and we had an evening out.
Hope you have an awesome time! Planning is almost (well, okay, not almost but a lot) as much fun as the trip.