Hello Marcel! Thanks for visiting the Disney Parks Moms Panel! This sounds like a nice family getaway, especially for the cousins! You have selected two fantastic Disney Parks to visit!
Magic Kingdom and
Disney's Hollywood Studios were the two parks that we visited most often when our children were the same ages as your son and niece will be during your visit this summer!
I am going to break your question into two. While you certainly can visit both parks in one-day, I am not sure it is the best parks planning strategy for a few reasons. First, you are visiting with younger children during a very busy and hot time of the year. August is usually when my family visits as well. I found that my kids needed an afternoon break back at the resort. That might be a nap, swimming at the resort pool, or just relaxing back in the room. Second, there are so many great attractions in both parks for children that age that I am not sure you will really be able to get in everything without having to make some pretty big sacrifices! We usually arrive at the parks for opening time when it's not very crowded, stay until 11:00, and then head back to the resort. We then head back to the same park later in the afternoon and stay until later in the evening or park-closing time. That afternoon resort break has saved us many mid-day "meltdowns" in the parks! Also, be sure to keep in mind travel time between the parks. This will take quite a bit of time from your day. Finally, if you were thinking of getting to Disney's Hollywood Studios first, in order to secure a Boarding Group for
Star Wars: Rise of the Resistance, and then head over to Magic Kingdom, my only hesitation would be that you could get assigned to a Boarding Group later in the afternoon or evening and that would throw off your plan for visiting both parks in one-day.
If you decide that visiting both parks in one-day is the best strategy for your family then I recommend you buy
Park Hopper tickets! These tickets will allow you to visit two parks in one-day for a reduced cost! It is a very good money saver! Also, I find that identifying which attractions are "must dos" for our family, prior to our park arrival, ends up saving us a lot time in the parks!
Marcel, as the chief planner for your family, you've got some important decisions to make! I hope that the information provided is helpful as you continue to plan your August visit!
Hakuna Matata! Jonathan G.