Hi, Katya!
You can absolutely leave a park and return after a nice mid-day break. Without the Park Hopper option, you need to return to the same park. It sounds like your plan is Magic Kingdom all day, so you are set to go with a base
ticket.
Except for pool hopping, you are welcome to explore the beautiful
Disney resort hotels, so if time allows, take some time to look around or simply relax in one of the spacious resort lobbies
When you return to the
Magic Kingdom after a relaxing lunch, you will need to re-scan your
MagicBand or park ticket at the touch points. Usually mid-day the lines getting in the park are very manageable so you should breeze through this and be back on Main Street USA in a jiffy!
Have a magical vacation in Walt Disney World!
Cathy