I have a photo that I took at a school picnic of a girl using sidewalk chalk. Her face is down and in a slight shadow. At the time, (several years ago now), I asked the mother if I could take her photo and use it as a reference for a painting. I did give her my card but I did not write down her name. She verbally gave me the ok to paint her child.
Is that good enough? The child was in a public place at a school event. She was not a student, but a younger sibling of one of the kids.
I could take liberties and adjust the facial features as to not look like her, but I think that's creepy for me to look at.