I need help doing a factory reset.

I want to do a factory reset on my daughter’s iPhone 6s, because I forgot my restrictions password. For some reason we can’t turn off find my iPhone due to parental controls. (We can’t even touch the iCloud settings in settings). Does anyone have a method that works?

P.S. We are trying to upgrade her iPhone 6s to an iPhone X using Sprint, and we need to have the restrictions password to activate her new phone. We are getting billed every day for having both phones, so help is needed ASAP.