I think that if the ILs were not planning to provide the food, that they should have mentioned that when they made the offer. "We would like to take DD on vacation with us. We would be able to cover the apartment rental and any outings, but would need you to send enough food for her for the week." Then you would know what you were signing up for. But, honestly, they didn't say that, because it sounds like they did cover her meals with no problems. I think that you may be reading too much into this. They were likely opening the door for you to send whatever food you WANTED to send, but they were not insisting on it.
And I wouldn't worry too much into how much (or little) fun DD had. Why does it matter? It sounds like she had a decent enough time and was returned well fed, not neglected, and was not scarred by the experience (even if she was anxious to return home.) Let her decide about any future trips. There are things that I decide to do that are just ok experiences. I may not have the most amazing experience ever, but it was pleasant enough to do and may even do it again. (I enjoyed my walk the other day. It wasn't life changing and I was happy to be home, but I had a pleasant time and would even consider doing it again sometime.) If that was DD's experience, I see no harm in that.