They can take their chances and withdraw, hoping the spot is still there in September, or they can pay some fee to hold the spot, or they can use it part-time to keep the spot open.
I ask for a 2 week non-refundable deposit(goes towards last 2 weeks of care) as well as 2 weeks per month holding fees if they're not using it.
If they want to use it, I have a 3 day a week minimum they must pay for both months.
Don't give away the spot for free for 2 months.
You could also try to fill it with school age kids while they're gone, and if you can't, then they must pay the holding fees.