With me ,if a parent wanted a place and it was not available until their date of start I would charge a deposit of 1 weeks fee.
However if the place is available before they are due to start and the family really do want to hold the place, the only way I could guarentee the place for them would be to charge a retainer fee of 50% of their fees to be paid weekly until they started. The place then is available for them to use when ever they wanted to and then the difference of what the fee should be would paid for the time used. Also 1 weeks deposit would be requested at time of signing contracts.
I started charging deposits several years ago at time of booking because sadly I had some families come and visit, sign up for childcare, have quite a few visits then do not turn up for their start dates. I offer quite a few free ''getting to know you'' visits and the deposit covers my time and stops me being messed around if a family then chooses to not come.
So in your case I would charge you 1 full weeks deposit (to be returned or taken off your last weeks care as long as notice is given and no fees are outstanding as per contract) and 50% of your weekly fee each week from the September until the January and then if you did send you little one between those dates , you would be charged the difference for each time you used your place. If you pay a retainer fee, that place is yours to use whenever you want to.
It does sound hard charging before you want your date but we could have a gap of 4 months were very rarely we can fill that space for a short period of time because we are holding it for someone, where we could have someone else using it straight away.
HTH :)