Hello, Marita!
It sounds like you have a perfect birthday planned for your little Mouseketeer!
Electric Conveyance Vehicles (ECV) Rentals are available in all four theme parks and Downtown Disney. Since you are planning to attend
Mickey's Not-So-Scary Halloween Party, you will need to rent a unit at the Magic Kingdom. However, quantities are limited and reservations are not accepted. It's also important to note that rentals must remain in the theme park where they are rented. Also, there is no guarantee that you will be able to rent an ECV because they are rented on a first-come, first-served basis.
I would hate to think that your mother might not be able to enjoy all the party activities, so you may want to consider renting a unit from a Disney featured provider. Many of the companies will deliver (and pick up) the unit to your Walt Disney World Resort hotel. This way your mother can have the freedom to use the ECV throughout the resort at her convenience. A list of Disney featured providers is at the bottom of the above linked ECV Rental page.
I hope this helps, and you make many spooktacular memories with your family! Be sure to stop by Guest Relations to pick up a Birthday Celebration Button for your son so Cast Members and other Guests can join him in celebrating his big day! Please let me know if I can be of further assistance as you plan your visit to the Walt Disney World Resort!
Have a magical day!
Missy