HI and welcome!!
The first, most important tip is this: know that there's no possible way to see it all and do it all. It simply cannot happen. So do what you can, see what you can, and chalk the rest off as stuff for your next trip.
Know that a pair of 5 year olds are going to want pool time in September. Don't fight it. After touring all day, that pool time is very attractive to everyone in my family.
Get to the parks half an hour before they open. Those first few hours of touring are golden, while the other people are asleep or enjoying their Mickey pancakes. We always stay in the parks till lunchtime, go back for that swim, and return at or after dinner.
Learn how both Fastpass and Photopass work.
Have a great time!!