Hello Mary! Welcome back to the Disney Parks Moms Panel.
Your plan to pack a lunch and store it in a locker until you need it is solid. However, you may find the perfect place to eat said lunch will require some walking.
The
All-Day Access lockers for Magic Kingdom are located to the right of the Main Entrance when entering for a fee of $7 per day + $5 key deposit for a small locker and $9 per day + $5 key deposit for a large locker. You are refunded the key deposit as long as you return the key no later than one hour after park closing.
With your locker options limited to the Main Entrance at Magic Kingdom, you have a few (relatively) close options for a shady or indoor spot for lunch.
The most idyllic location would be to take your lunch to
the Hub in front of Cinderella Castle and find a spot on the lawn area for an impromptu picnic, however shade will be limited.
If you can find an open umbrella table in front of
The Plaza Restaurant, you will have both a comfortable seat and shade from the hot Florida sun.
I recommend taking your lunch to either the second floor of the
Walt Disney World Railroad(facing Town Square) or the alleyway down Main Street, U.S.A. before the
Crystal Arts store. Both of these locations provide you with a reprieve from the crowds, a few benches to rest your weary legs and some shade while still remaining relatively close to the lockers.
Have a Great Lunch!
~Todd