T.C.
Um. Your mother chose to change her appearance to make herself feel better. They love their grandmother. If she had been banged up in a car wreck you'd explain that she she had bruises and scratches from that. Explain now that Grandma had the doctor fix some things she didn't like, and move on. They won't care. Take them over in the evening to watch a movie with Grandma. The lights could be lower so that the bruises won't be quite so stunning maybe, but be honest and let her answer their questions.
This might be a good time to talk to your kids about appearances and how we're still the same person on the inside... in a possitive manner. If, then later, something bad happened to someone they loved, like an illness or bad burns, they would have something to relate it to... My girls went through my mother's cancer...loss of hair, change in color, etc... just fine.