In my personal opinion putting chemicals in the hair of any child under 10 is a form of child abuse. My mom put a curl in my hair in first grade, then continued for years. I remember throwing up many times the smell of chemicals made me so nauseated. Didn't matter. Next came the years of relaxers and scabs on my scalp.
Do you want your child to have a chemically burned scalp? Do you want to put her through that every few months?