If you charge them half the rate it is like both get a deal and when you think about it you aren't really losing that much because you aren't giving the child any food, craft supplies or using toilet paper, wipes, washing her bedding etc. for three weeks and the upside is that since the family feels they got a deal they stay loyal. But make it clear that it is an annual thing such as for only for a two week period so for three weeks it would be one week at full rates and half rate for additional weeks.