What about AoA family suite, a bed room with a door, queen bed with a full bath, and pull down the 12 year old can sleep on and use the pull out for DD, either rent or bring a pack and play for the baby or You could let your DD in the bedroom and set the pack and play up in there so if she needs to take of the baby in the middle of the night it would be easier maybe not wake you and your son up. Kitchenette and a second full bathroom, really nice when everyone is getting dressed, and room for the baby to move around. We stayed at AoA and it was great, we stayed there on our girls only family trip, my DM, and DD and myself and it was soooo nice to have an extra bathroom and room for everyone's stuff.
With your trip planned for August which is hotter than hot and the humidity is miserable, you will want to spend some time in the pools, and relaxing out of the heat with the baby will be necessary. So some more room will be important also there are 2 TV with 2 kids different ages that might be another factor in your favor. Another thought with the baby, having a door so she can nap might be to your benefit, the parks are open late so taking a break mid-day would be great.
Make sure to keep the baby hydrated, really all of you will need to drink triple what you normally do. Water and sports drinks are a must, I would suggest your daughter talk to the pediatrician on drink choices for the baby.
Have a magical time
