Hi Nicole! First time experiences at Walt Disney World are truly one-of-a-kind. It is definitely an experience that you both will remember for years to come.
I traveled to Walt Disney World last August for my son's 5th birthday, and it was moderately crowded. Since you are traveling in late August, the crowd level should be lower because most children have returned to school by this time. It also depends on which park you are in. It was extremely hot last year and rained almost every single day that we were there. I suggest that you buy a poncho for each day that you plan on being out in the parks. I bought some dollar ponchos and saved so much time and money by having them stocked in my backpack.
Since I was on a budget, I planned 'surprises' for my son in advanced. I bought him some fun Disney coloring books, stationary, stamps and stickers for the plane ride and he was so excited. I also bought Mickey Mouse decorations from my local supermarket and we decorated our resort room window.
Sometimes you never know what a child is going like or dislike, so when we got to the parks I was pleased to find that my son's favorite things were the pressed coins and character autographs. They are both relatively inexpensive and they are souvenirs that he can continue to collect on future trips to Walt Disney World. I also suggest reserving some Character Dining Experiences so that he can meet some of his favorite Disney Characters while enjoying a delicious meal.
Chef Mickey's is my son's all time favorite.
I'm running out of space, so if you need any more suggestions please feel free to stop by and ask some more questions.
Have a magical day.