I hope my children are as wonderful as you when they grow older -- a birthday trip to WDW!! With the size of the parks and resorts at WDW, there is a lot of walking. Your mother may feel more comfortable in a wheelchair to keep her off her feet. Wheelchairs can be rented at the parks and Downtown Disney. For information regarding rentals, you can click
here. Another option is to rent an Electric Convenience Vehicle (ECV) that will propel itself. For more information regarding these rentals, you can click
here.
You may also want to rent one from outside WDW so that you have one as you move from the parks to the bus stops/parking lots and within the resorts (many are quite large). While I have not rented one,
The Complete Walt Disney World 2011 travel guide recommends the following companies: Apria Health Care (apria.com), Care Medical Equipment (caremedicalequipment.com), Turner Drugs (turnerdrug.com), and Walker Mobility (walkermobility.com). And please know that Disney is OUTSTANDING for assisting guests in wheelchairs; my mother is in one, and we have never had difficulty (other than my brother insisting on popping wheelies with her in it).
I'd also encourage you to include a notation in your reservation about your mom (her birthday, too!) and the room preference to best help her, such as close to a bus stop, first floor, etc. Just note that requests aren't guaranteed.
Wishing you a magical trip!