Everytime I have cared for a teachers child I have charged a 50% retainer upfront. The last day of care before the summer break, the parent has to pay me the 50% for the entire Summer and I must have post dated cheques for the first day of regular care in September in hand. Nobody has ever had a problem with this condition. If I were you, regardless of what you decide to charge, I would get the money for the whole summer break as one payment at the start to prevent being screwed over.