Nannying during the summer?

I have a been a nanny and I say this to you in all seriousness. Never, ever, ever! put a set rate per week on nanny work unless you have a contract that says the pay rate applies for anything up to 45 hours, and any time after shall be considered overtime and paid at $x/hr. So many nannies get comfy with a family, set a weekly rate, and suddenly the parents are gone 60+ hours a week without asking if you mind staying super late.