I was there in late September/early October last year. The heat was brutal. Its kinda better early in the morning and at night. It also tends to rain in the afternoon. Always carry ponchos/umbrellas. The downpours can be quite intense, and I've found a poncho/umbrella combo works best. There's a Walmart that's pretty close, and you can stock up on umbrellas, ponchos and water there.
I could be wrong, but you might be there during the Food & Wine Festival at Epcot. The best time to visit is for lunch. Lines are crazy for dinner. Personally, I was extremely disappointed in the food. The smoked salmon and potato pancake from Scotland was the best.
I believe you can still do a switch off at Dumbo. One parent gets in line with the kids, and the other parent also gets in line. That way the kids get to go twice. Its been a very long time, so I don't know if you can still do this.
If you're Star Wars fans, they have meet and greets now in Hollywood Studios. I think its Vader and Chewbacca, but I might be wrong. I know they just added BB-8.
If you decide to do any of the parade/fireworks fast passes, be prepared to stake a claim to your space. Ideally, be near the fence so the kids can see. We had a horrible experience with some folks who insisted on standing in front of us for the whole show, and we missed a lot of it.
You can also get a great view of the parades & fireworks at MK at either the train station or right by the fire station. They were refurbishing the train station in January, and I don't know if its open again.
Also, you can either walk or take a boat between Hollywood Studios and Epcot. Just exit through the International Gateway from Epcot. Just keep an eye on park hours, since you don't want to get stuck. If you do get stuck, you can always go to one of the bus stations at any of the Epcot Resorts and get a bus to Disney Springs. Then from Disney Springs you can get to your resort. Be sure to keep an eye out for busses at Disney Springs, and have at least one member of your party visible to the driver. In the past, they've been a bit sloppy about pickup there.
You can also get a monorail from Epcot's main entrance to MK. You need to go through security at Epcot to do that, though. The only park that's a bit difficult to get to is Animal Kingdom. Its only accessible via bus or car.
Personally, I've had a lot of issues with the MyDisneyExperience app. Especially for making FastPass selections and dining reservations. I just use my laptop.
Be aware that you'll be in Florida during hurricane season. Keep an eye on the forecast every day and have a plan, in case things get bad. We were there right before Hurricane Matthew hit. We made the decision to get out, so I don't know what its like to weather a storm down there. I do know you'll be restricted to the resort itself. If you do find yourself in that situation, you might want to make a food run so you're prepared in case the worst happens.