Welcome Fintini! It is exciting to start planning for your first visit to Walt Disney World!
July 1-3 will indeed be a busy time at the parks since the US holiday, Fourth of July is on July 4! Guests will come that week to celebrate the holiday and take advantage of vacation time from jobs etc. It will also be quite hot in July and the average temperatures will range from 73-92 degrees Fahrenheit with average rainfall over 6 inches. In the summer months it is usual to have an afternoon rain shower or thunderstorm so you need to be prepared for that. Disney sells inexpensive ponchos to wear and I suggest if you are caught in a rain to just buy the ponchos and stay in the parks. The humidity in Florida in the summer will be what is the hardest part of the weather to handle.
I would certainly consider at least two of your three days to be spent at the Magic Kingdom. During busy times the lines will be longer and this way you can feel less stressed knowing you have two days to see as much as possible. On the third day you might want to consider another park and my suggestion would be the Animal Kingdom. I am sure your four year old son would love to see the animals in the safari ride, birds in the Flights of Wonder attraction, the musical shows, and the train ride to Rafiki's Planet Watch where he can brush the sheep and goats.
Be sure and obtain your dining reservations as soon as possible for the parks as the restaurants book up quickly. You can do this on line at the website,
www.disneyworld.com or by calling 407-939-3463. Let us know what else we can do to help you plan this first visit!