I would use one zip lock snack bag for every day you are gone and put the food for each day in there, plus a couple extras in case you do not get back in time.
Just let them know that if they miss a day do NOT feed double.
In addition to the clean water filter, or what ever you need to do.
The worst times I have had in my tanks is when people come over and feed the fish, or don't and then put it all in at once. Had the most disasters with my friends, pet sitters, even those who have their own fish!!!!!
Make sure you have a decent amount of dechlorinator in case of emergency.
What should they do if the power goes out Plan?