I have 2 school aged children who eat a lot and that is excessive IMO. I too would send a lunch box, and I would probably make extra food for the night before (say dinner) for hot lunches, and make a container that can be reheated in the microwave for each child.

It really irks me when people take advantage of their employers, be it babysitting, an office or what not. If it was something you offered to do then I think it's rude to be expected to feed all the children. I'll say this in the past when I have left my kids with people I often leave enough food for all the children even if I'm paying... but it's not something I need to do every day!.