Hey there, Judith!
Thanks so much for stopping by the Disney Parks Moms Panel!
Renting an electric scooter or ECV at the parks is on a first come, first serve basis.
There is a chance that one may be available for you once you arrive at the Magic Kingdom, but it can't be guaranteed. ECVs at the theme parks can only be used at the theme parks and must be returned before exiting.
One sure way to have an ECV for your magical experience in the Magic Kingdom is to pre-book and pre-pay for the scooter at one of the Orlando area rental companies. Many of these businesses will deliver and pick up the rental at your Orlando area hotel. If you are staying at a Walt Disney World Resort, you can have it delivered to Bell Services at the time specified. This is also a good idea if you will need to scooter for multiple days! Having the ECV available to use around your resort may be useful, as well!
Here's a list of local ECV vendors.
Best Price Scooters and Strollers
Apple Scooters
Scooterbug
CARE Scooter Rentals
Buena Vista Scooter Rentals Enjoy your time at Magic Kingdom! If you have more questions, please visit with us again soon!
Happy planning!
Korliss