No...you are fine. There are 52 weeks in a year.
What calendar is the parent using!
Your formula means even payments over the year unless you credit them back for a few vacation days.
I have had people question it too tho'. They tend to think the monthly fee should be the weekly fee X four.
I've had to actually pull out a calendar to show that some months have 5 Fridays and do they expect me to just go without pay for that Friday???? That usually hushes them up!