I find summer care hard. Children older so it mean finding activities which engage school age child that younger one can also do - or running two program. Also student eat more than toddler so that a factor to think about with fee.
In term of them not need care some weeks, too bad. If they want the flexibility of knowing they have care when they need it, they need to pay for the space for the duration of summer like every other client. I sure my teacher client who pay my bill year round would think unfair if a temp client got preferable financial rate then they did. Yes, I would charge for the day they away. They not doing you favour by filling 9 out 10 weeks, you doing them favour by extend program to include their child, by adapting program so it fit their child, in reserving days for short term client and by them not have book summer camps which be more expensive.