I read a book in school (early 80s) where the entire world was paved and everyone traveled in personal wheeled carts. There was a little girl who fell out of her own cart and discovered she had legs. There was a small patch of grass or some sort of greenery near where she fell but it was the only patch around.
This might be "Revolt of the Pedestrians" by Dr. David H. Keller as mentioned in this answer to Short story about a future where people spend all their time in cars.
“The Revolt of the Pedestrians” is set many centuries in the future after humans divide into two species – the pedestrians and the automobilists. Automobilists have shrunken legs from always using automobiles or powered chairs called autocars. Eventually, the overwhelming majority of society become automobilists and decide pedestrians are anti-progress — a threat to society. They are outlawed, killed and assumed extinct.
One of the main characters is Margaretta Heisler, a young girl who was born with functional legs despite being part of a prominent Automobilist family. I haven't found any reference to her tumbling off and realizing she has legs. There's a large portion of the text on Google Books which is unavailable.
As an addendum, there is, after all, a copy on the Internet Archive and the falling part does not match up. Margaretta is born with functional legs, and is given training to use them by her bemused, but loving, family.