Hi Ruby,
Please do not worry! I think that with a little advance planning and knowing what to expect, you are going to have a wonderful trip.
First, I want you to know that you are going at a time of year that my family often travels to Walt Disney World. As you get further into August, many schools are already back in session, which makes crowds lower than they are at the peak of summer. Yes, there will still be a lot of people at Walt Disney World, but likely not as many as there were in July.
August is one of the hottest months in Florida, with an average high of 92 degrees Fahrenheit. This is something that we cannot change - but we can plan for it. My family likes to get up early and head to the theme parks early. It is so much cooler early in the morning! We often have lunch at the theme park, and by early afternoon we are heading back to our resort to take a swim in the pool, which means that we miss the heat of the day at the theme park. Then, if we want to, we can go back to the park again later at night when it cools off a bit. By the way, the tip to go early also helps with crowds! Since most people will not get to the parks until later in the morning, by arriving as early as we can, we can do more attractions with shorter lines.
Also, and especially since your children are so young, take advantage of the
Baby Care Centers. These are located in each theme park and offer a place for you to feed your children, and for them simply to unwind in air conditioning. I used the Baby Care Centers a lot when my daughter was a baby, and they were wonderful.
I really want you to not worry about your vacation, Ruby! Please do come back and let me know if there is any more information or advice that I can give you to help you have a magical vacation.