Bread of your choice
I get the avacodos 4 for $1 at a farmers market down the road. (They also have 2 heads of lettuce for $1 but I often get lettuce free from my mother.) You only use half per sandwich so 4 will feed 8 people. Using steak tomatoes you can make everyone's sandwich, depending on the size of bread, with just 1 tomato. I can get 1 steak tomato for $1.
Spread some mayo on the bread and top with lettuce. I like Romaine or Iceberg but you can use whatever. Top with sliced tomato. Top with the sliced Avocado. If you don't know how to slice an avocado you cut them in half and remove the pit. Slice through to the skin length ways. Gently scoop out with knife, fingers or spoon. Sprinkle on some italian dressing. You can also top with bacon, when we use it was use turkey bacon. Add a cheese of your liking, we get this at Costco so we don't spend as much per serving. Onions can also be added, but kids usually don't like onions. I use Jewish Rye but you may find you like other breads. Since it is a messy sandwich you might want to try it in a pita pocket. This is basically a salad on bread.
Another cheap lunch the kids like is Mac and Cheese with Tater Tots and Bananas. I have to make this once a week for me own sanity, they get withdrawals if they don't mac and cheese at least once a week. You would think their blood was made of Valvetta. I guess instead of Tater Tots you can do roasted potatoe slices to be a bit cheaper, but tater tots is easier. And we all know, time is money!