I leave my kids with their "cool" auntie too. Never thought to leave a list saying "no coffee" at 1 year old. I'm not suprised you laughed.
Yeah, I would be a little insulted if my sister thought I needed to be specifically instructed not to give coffee to an infant!
We left DD with my parents overnight when she was 14 months old and I think I did write up her routine for them (i.e., her usual nap, snack, and bed times). I intended it as a helpful guide, not as instructions for them to follow. I think that sort of approach - giving them general information about his favorite foods and those that don't agree with him - is the way to go, rather than mandating a certain diet. But you have to be willing to accept they might not follow your suggestions.