Opportunity for her to learn something. Coach her on how to approach the parents herself, where she explains what a reasonable and fair rate would be expected by her age group/weekend/etc (when not discussed up front by the "customer") and why age shouldn't determine whether the customer gets to pay cash or some other tender.

She might see standing up for her concerns in an appropriate way can be positive for everyone, gain confidence, gsin respect of those parents, plus have more $ in her pocket which she already earned. Just a thought