She got it and it burned her scalp
Do you know why her scalped burned?
I might have mixed it with shampoo
If you mixed something in with shampoo that caused it to burn someone's scalp, that is very wrong since it put someone's physical health in danger. You should never put even potentially harmful substances into people's personal care products as it can cause permanent injury to them and get you into legal trouble.
It was an accident, I didn't mean to do anything bad.
I can understand that you never meant to cause any harm, but your actions were highly risky, and a court would find you liable for the harm caused.  Household chemicals, even when they are safe by themselves, can react in unpredictable ways, so you should never mix them unless you follow the instructions carefully.