Greetings, Mathew! I'm so happy to welcome you to planDisney and thank you so much for trusting us with your question.
It sounds like you have a wonderful plan already in place to enjoy your 1 Park per Day ticket during your visit. One of the things I love most about Disneyland Resort is the proximity of the theme parks, the Downtown Disney District, and the hotel properties of Disneyland Resort and surrounding Good Neighbor Hotels. It makes it so much easier to take that mid-day break when you need to rest or refuel before enjoying the theme parks in the afternoon and evening hours!
To answer your question, Mathew. Yes, it is absolutely possible to enter the theme parks with your valid ticket and park reservation, leave for a bit of time, and then return and re-enter on the same day. Technically speaking, the terms and conditions of your ticket state that park reservations are subject to capacity limitations, and park admission is not guaranteed even with a theme park reservation. However, this has happened very rarely, and only on the busiest park days like major holidays. In fact, it has not occurred in the years since the theme park reservation system was put in place, so I would find it very unlikely to take place during your next visit! You can always ask a Cast Member before you exit the theme parks, just to be safe. But, I truly hope you can enjoy that time in the middle of your day to take a break and refresh before embracing the magic for the rest of your evening!
I hope this puts you at ease in advance of your next visit, Mathew. If we can lend any help in the meantime by answering any other questions, please feel welcome to reach out to planDisney again. I'm wishing you wonderful, unforgettable memories with your loved ones!
Magically yours,
Tishra