Hey, Daniel!
Thanks for your question and for visiting the Disney Parks Moms Panel! Helping guests' to plan their Disney vacations brings us great joy!
Lockers can be rented at each theme park for $8/day for a small locker and $10/day for a large locker. A $5 key deposit is required for either size and will be returned to you once you are done using the locker. These lockers are only available on a first come, first serve basis, so try your best to secure one as soon as possible upon arrival at the park. Once you rent at one park, you can take your receipt to any other park, show your receipt, pay the $5 refundable deposit, and use any available locker.
At Epcot and Disney's Hollywood Studios both have coin-operated lockers available at the bus information booth. Only quarters are accepted for these lockers.
At the Magic Kingdom, lockers can be found inside the park, just before you walk under the train station!
Enjoy the magic and make lots of memories!
Happy planning!
Korliss