Eveyone will do their own thing. I think it would depend really on who it is whom is paying for the childcare. Some parents may not want to pay retainer fees during the holidays. Who is your target market? Some people are paid throughout the year, such as many teachers, while in other professions if you are only paid for the actual hours you work, rather than calculating annual hours and splitting pay over the year.
What you could do is agree an annual salary, rather than doing it hourly. Then you can split that into monthly payments, which would mean you get paid during school holidays. Working out what happens if the mindee leaves part way through a year however can be tricky, so would need to build in a clause to the contract that deals with contract termination and calculation of fees owed/refunded.