If the space is available now I would ask for a retainer, as you could "sell" the space to someone else. If you aren't desperate to fill the next 4 weeks or don't think you could sell it to someone else you could charge a deposit, which could be used towards their childcare costs when they start. I negotiated a 50% retainer for my parents, as although can't really afford to give the space away, understand that paying for 3 months of childcare which they don't need may have put them off. Obviously if they pay the retainer they can use the space at any time.
In this case I charge both, The deposit (I charge the equivalent of 2 weeks fees) to secure the place (& I keep this till the end of their contract when it is credited towards their final invoice, that way I always have 2 weeks of their money in case they do a runner!) and the retainer (50%) to cover the place whilst it is booked by them but not being used, to cover my loss of income from keeping the place for them. I don't normally charge a retainer if they start within 3 weeks of signing the contract though as this time is often used by them anyway for settling in sessions