Hi Richard,
Thanks for visiting the Disney Parks Moms Panel!
You can rent an Electronic Conveyance Vehicle (ECV or scooter) right at Walt Disney World. Information about
ECV rentals at Walt Disney World can be found online. That site also lists five outside companies who supply ECVs for Walt Disney World guests. All of their phone numbers are listed on the site to contact directly if you have questions. I also know that others have had good experiences with Walker Mobility, who can be found online at walkermobility.com. Thanks to my fellow Moms Panelists Beth and Anna for their help with this information.
How nice of your grandkids to want to do something nice with you at Walt Disney World! One idea that comes to mind, since you are Disney Vacation Club members, is that you could all go to the Top of the World Lounge together to view the nighttime fireworks from Magic Kingdom Park. You would show your Disney Vacation Club member card, as well as your current Key to the World Card, in the lobby of Bay Lake Tower at Disney's Contemporary Resort. From there, you take an elevator to a lovely lounge where you can get drinks and snacks, and there is a large outdoor patio for viewing the fireworks.
Another option is to take a Specialty Cruise at Walt Disney World. From specialty cruises including private dining to cruises designed to allow you to see fireworks, these are a fun way to celebrate and spend time together.
I hope that this information is helpful for you, Richard. Enjoy your time at Walt Disney World with your grandchildren, and please do not hesitate to ask if you have any further questions!