Up grade, so you do not arrive frazzled, at either end.
As suggested up thread, "We are doing this tomorrow, what are you doing?"
If you are heading to the same place, say "We will meet you there",
'Shall we meet for lunch,ring us when you start thinking about it" then everyone can then go at their own pace.
buy local pay as you sims, when they arrive (some hours later) then you can all meet up.
book a days air boating (find a decent company, that will show you the amazing bird and wildlife), no one silly enough to take a three year old on an air boat,
The parks you will be on different rides,
It will be hot, so he may nap in the afternoons,
Make sure you have separate hire cars,
the beaches are huge,
it's really cheap to eat out, cheaper than cooking at home, so hopefully by the time you are back in the evening he will be in bed, drink lots of wine, leave early in the morning so as to make the best of attractions, if you arrive at the parks really early you beat the traffic and get to park by the entrance not a mile away and have to get the transport.
make sure you discuss before you go, how you expect to be passing ships as obviously trying to do everything together will never work, and it would be unfair on them having to make a three year old keep up.
hopefully you will be able to sit and enjoy a glass of wine in the evening by the pool and swap stories.
take ear plugs, and eye masks, Florida architecture is usually generous, so you should't get disturbed in the mornings.