The most important thing is to determine your priorities and get there as soon as the park opens. Do NOT go on an early entry day at MK or it will be mobbed and you won't get to go in early. I would focus on 1 or 2 lands in the AM that are most important to you, use fastpasses where you can, and then see the "lesser" attractions (i.e. the ones that don't usually have long lines) later in the day. Oh yeah, and don't forget to rest at some point during the day! And resting can even mean the Hall of Presidents or Country Bear Jamboree, where you can sit for awhile.
You could get out of the park in the afternoon and go to a resort for early dinner and then come back into the park for the evening activities. It's nice that in July the park is open fairly late.
Our first time at Disney my DD was 7 and it was our most magical trip. She was just the right age to do all the walking and had no tantrums and made all the height requirements and was only scared on Alien Encounter. Perhaps you don't want to do that one.
I say go for it!