A lot depends on demand in your area. If you can fill the spots then tell them that and take your chances if they pull out. Also what is the reason for going part time in the summer - teachers get a full years worth of pay just given over 10 months so they should pay for a full year of care either with summer fees or higher Sept to June fees. Whatever you do make sure it is a set amount per week to hold the spot and then an amount per day they come so you are guaranteed the income such as half the rate and then the balance for a full day for any day they come.