Welcome to the boards
I would back up what Miffy has said, particularly with regard to ...
Arriving Early - Its your holiday and it can be a pain to get out of bed but on a theme park day, it really pays to get there
before the scheduled opening time. You'll be able to get a lot done before the crowds hit the parks. If possible, pick up park guides (loads of shops, restaurants + the parks themselves have them) so you can look at what you'd like to ride and get a basic feel for the layout of a park. Of course, a book like
this one is excellent for this stuff. Arriving early is
vital if you plan to visit a water park. As we found at Blizzard Beach, if you arrive just 45 minutes after the park has opened it can be nigh on impossible to find a free chair or sun lounger. We didn't find anything free! Get to a water park early and bag a couple of shaded sunbeds/chairs with your towels ASAP!
Fast Passes - Once the park begins to fill up, make use of the FP system so you can bypass (most of) the queue into a ride. It's very easy to use and saves loads of time. On the same line as you have a child, if they don't want to ride a particular attraction then ask the ride staff about the child/parent swap systems. Most of the rides have this in some form or other so you only queue up once. An adult and the child can then wait for the others to ride, then the waiting adult can go on without having to queue.
OK, those are two millitary type exercises but remember this is your holiday and you want it to be fun. Try and plan in a few rest periods/days - make sure that people get a lie-in once in a while. No one can tell you how to manage the child in your group, its up to you to 'read' them and see how they're feeling. If they're getting tired from the walking, and believe me, you will do a lot!, try and slow the pace a bit. However, if they're anything like mine who were like a pinballs buzzing around the table ...
I'm sure you'll have a great time. Relax. Feel the warmth of the Floridian atmosphere and soak up the hospitallity that awaits you.
Enjoy!