Were I in that situation, I would explain the next time: please be sure that the food stays in the kitchen and the kids get their dishes to the sink. Don't say "that last time was a mess"... just go forward proactively. She needs to be taught.
I have noticed that our adult sitters generally do tidy up because they are used to looking after themselves. Kids, not so much. I tend to let things slide if I know that my son was having a great time and they were playing.
So, for next time, I'd leave a note:
Please make sure that the kids keep the food and dishes in the kitchen. They are used to bringing their plates to the sink, so they should be cooperating with you on that.
Please make sure that the kids clean up the activities they finish before moving to something new. This is a usual house rule around here, but they sometimes forget because they are excited to see you...
Etc etc. And I never expect sitters to do dishes. I did, when I was a sitter/nanny, but I don't assume that I'll be coming home to a clean kitchen. And I'd rather they be playing with Kiddo.
I do draw the line at sitters who use the iPhone as a way to keep my son busy. We have rules about media and I think that if the sitters don't know how to play with a kid, they probably are in the wrong job, y'know?