Hello Alicia and welcome back to the Disney Parks Moms Panel!
The simple answer to your question is: you
do not need a Park Hopper Pass to leave a Park and return later in the day. You are correct that you only need the Park Hopper Pass if you plan to visit more than one Park in any one day. This ability to leave the park and return later is extremely helpful when you have little ones and want to take a break from the action.
While single Park tickets work well for many families, my family prefers the freedom that comes with the Park Hopper option. More than once we have found our evening plans foiled by weather or other unavoidable issues. For example, we had planned to attend
Fantasmic! on the day we toured Disney Hollywood Studios. When the clock struck about 7:00 pm (about an hour before the show) we realized that our youngest would not last that long without a tremendous meltdown. Since we had the Park Hopper option we knew that we could change up our evening plans for a different night and return to
Disney Hollywood Studios to catch the show. For us, the convenience is worth the added cost. It really is up to personal preference.
I hope this helps with your planning.
See ya real soon,
Anne