JanetDI always make sure we have plenty of water and salty snacks. Like others stated, cool clothes. I wouldn’t worry too much about her not eating -- it can be hard to have an appitite in the heat. Pretzels can do double duty -- provide salt and something in the tummy.
If you ever figure out how to get your daughter to eat at such events, let me know. Regardless of what I do, my 3.5 and 5.5 won’t.
Janet