My situation is a little different in that my son was barely 3 years old when I met my husband (who is a C4/5 complete and uses a chin controlled power wheelchair). So he never knew him not in a chair, but still, his chair looks like something out of Star Trek and can be very intimidating even to adults. But what I did was act like nothing was different. I introduced them, he looked at the chair with puzzlement, walked around it, staring, looking ... then he climbed up on his lap, pointed and said "There!", after which my (now) husband drove him "There!". :) :) :)
Since your daughter already knows your mom as ambulatory, I would just tell her the basics, that something happened in her body that means her legs don't work anymore, so she uses a wheelchair instead of legs. Then just let it be and let her work it out. Be sure that YOU don't convey any fear, awkwardness or embarassment. Kids are laser sharp when it comes to emotions, so if you are comfortable, the odds are that she will be too.
Last word of advice - make sure when you tell her about your mom, reassure her that it won't happen to her. Kids sometimes think that if something bad happens to someone else, it could happen to them. So you could say that grandma had a problem in her spine that caused her to use a wheelchair, but that is not something that will happen to her, that her spine is great - then maybe do back bends, or stretches or wiggle/dance to "show" her how great her spine is.
Which reminds me of a very precious story. When my son was about 5, he was in a phase where he really wanted to be like my husband. He was at his office, and my son was playing in his office. He looked at my seriously and said "Mom? When am I going to get my wheelchair?". He thought he'd grow up to be like my husband, so he thought that meant he'd need to be in a wheelchair when he got older. Heartbreakingly sweet, bittersweet - and gave me a glimpse into how kids think.
