Hello Ashley!
Wow! Your MAGICAL Walt Disney World Vacation is about to begin! You all are going to have an incredible time!
I have five kids under the age of 12, so I totally understand your question. I can honestly tell you that in all of the trips we have taken to Walt Disney World, we have ALWAYS chosen to not rent a car and use the
complimentary Disney transportation. You have also picked a GREAT time to go to Walt Disney World! From my experience, the crowds are much lower in late August and early September. While there will be times you will have to wait for a bus, boat or monorail, there will be many times you will just walk right on! It's also nice that the Disney Transportation System drops you close to the entrance of each Disney Park. If you drive your car, you will still have to park, unpack the car, hop on a tram, and walk to the Disney Park entrance.
The bottom line is that I really think your family will be very happy using Disney transportation! If you have other questions, please let me know!
Have a MAGICAL day!
Wes