Do they know what your current daily rates are? A very simple letter saying due to an increase in the cost of food, supplies and learning materials it will be necessary for all children in care to pay the same daily rate for full day care. Therefore your new rate for summer care and any full day of care during the new school year will be $XXX per day per child. Come September the before/after care rate will be increased to $XXX per day per child.

Since you don't have a contract you need something in writing so you could give them two copies of the rate letter asking them to keep one for their reference and one to be signed and returned by a specified date - about a week away.

Then if they are not happy with the increase they will move on and give you time to replace before summer.