Mine in August too!
We did it through the website of Discovery Cove.
I suspect you will just need to go on to the website for the character meal (we never did these as my DS's weren't interested).
If you have booking references I cannot see why it cannot be done.
Are you going to Seaworld? If you do and want to see the
Shamu show, you need to start queuing for that about an hour or more before the show starts.
Aquatica (opposite Seaworld) closes it's gates around 10.30am as it will up to full capacity, it reopens them about 3/4pm. We used to get there really early (like 7.30am), the traffic will already be queuing to get in.
Typhoon Lagoon (IMO) is the best waterpark.
Do the rides in the parks back to front. Ie go the ride furthest away and work backwards. There will be less of a queue and believe me in August they will be huge (90 mins plus).
Single rider option is the way to go on the rides that just you or DH or one of the children want to go on. You more or less get to go to the front of the queue to fill a gap.
If you have a villa/apartment, put small bottles of water in the freezer the night before you go to a park and then pop them into the bottom of your rucksacks (sandwiches on top). It will keep the sandwiches cool and by mid morning the ice will have started to melt and you have for nice cold drinks.
Mexico is also lovely, if you do your third week there, visit Chichen Itza. Pick a day though where there could be a good chance of rain/cloudy as it is inland and very very hot. The rain (which probably won't last long and it will still be warm) will be a welcome relief.