I would probably let them pay the two week deposit a month apart or at least two weeks as in one with each paycheck since I know a lot of families live paycheck to paycheck or do a $200 a week till it is paid so it is spread out over the month. It is still early for Sept so there is time to allow this and make sure that anything paid is non-refundable if they stop making the payments.

If you holding the spots then you could also ask for a monthly payment till then but if they are a replacement for kids already in care the deposit is normally considered enough.