If the spot doesn't open until close to when the clients want it I:
Take a deposit, have then sign a contract, have then sign my Spot Reservation Agreement (which states that if the spot opens up sooner then they have to either pay a holding fee of $100/week, start care early or forfeit the spot.)
I also have a waiting list for those who are not ready to sign on.
If the spot is open now I would:
Get a deposit, Have contract signed, get a holding fee of $100/week for 13 weeks then full fee until the date they want to start.