I do it differently - again.

One week deposit per child with contract. I don't consider a place taken without both of these. Deposit is to hold the place and non-refundable. They get it back as their final week of care which if someone is being a pain about paying their notice period, is useful to have.

I won't hold a current vacancy for more than a month unless the parents pay a retainer for it, normally 50%. Of course, they can start early but would pay full rates then.

I won't hold a place for more than 2 months unless it's for my convenience - i.e. I've just filled my places for Sept 2015 with deposits and contracts so of course, those parents won't be charged a retainer as the place won't be available before their start dates anyway.