I have school on a couple of days of the week and when I do, my daughter's grandpa picks her up and either he or my great aunt watch her after school until I get home. Like your mother, my father in law and my aunt are not ones to cook and usually they let her have anything she will eat that is easy for them which has in the past meant alot of junk. I found that if I make a little extra when I cook after I get home I can save it in microwavable containers for the nest day while I am gone. This way I can just give what I have to grandpa or Aunt Marge asnd they can pop it in the microwave for a minute or two when she is ready to eat. Makes it easier on them and I know that she is getting what I want her to eat. Casseroles reheat really easily and are easy easy ways to sneak in all kinds of veggies.
Another option - Sandwiches are easy and can be made with lots of healthy ingredients, you can have them on bread, in a pita, or flat bread as a wrap. You can use hummus,meats, all kinds of veggies, even slices of fruit. My daughter loves it when I spread a little cream cheese on a tortilla cover it in spinach, turkey, tomatoes, a little cheese, roll it up and then cut it into little pinwheels. Tuna salad works really well this way too and even peanut butter and jelly.
I also make little baggies of cut up fruit and veggies and then add a little cup of something to dip them in like peanut or almond butter, ranch dressing, hummus,(you can get really small plastic containers at the grocery store) works good for snacks or to add more nutritional value to a lunch of peanut butter and jelly sandwiches.