That would be frustrating for sure. However, if she was offered something at your place and didn't eat it, is it maybe that she just isn't a breakfast eater? Did she say she was hungry? If a 7 year old told me she was hungry and I took the time to make her something to eat, she would be eating it. That may seem harsh but at that age, they know if they are hungry or not! Why say yes if you aren't going to eat it? (I'm more lenient with the younger kids but not a 7 year old)

I would talk to the parents again and explain that breakfast is needed to be eaten at home before coming or you will have to charge an extra $2-$3/day because you are feeding her at your place (whether she eats it or not, it's still being supplied).

If she isn't a morning eater, I would just leave it be and she can eat at morning snack.