As a regular substitute teacher for elementary , I see a lot of options.
First as an address to the "credit" issue. ALL children have an account with the cafeteria. ALL children have a pin. This is used when everyone buys lunch at school. We go through a website where the monies can be replenished. If money is not replenished and a child is coming through the line, they get a note to remind parents to replenish the account and a cheese sandwich.
That said, parents have first call on what children are allowed to purchase and not. Example, one kid routinely gets 2 pizzas on pizza day... he is turned away everytime because there is a note on his account to have 1 pizza with the other courses served. Same goes for snacks. Snacks are only offered to the higher grades, not K. There is no option for ice cream, etc.
I get where you are coming from with "credit" but this program works well for children because most are not capable of keeping up with daily cash. The lunch ticket back in the old days is the same effect. Of course, debt free and no credit is to be taught but the lunch account which is YOUR money is different.
Now for options... I see a lot of soups, noodles, hot rice, etc in a thermos. I see a lot of yogurt, veggies with dip, meat snacks, cheeses, peanut butter (I know many people frown upon that but is is a good source), leftover pizza wrapped in foil, cold chicken nuggets, gogurt is popular but it is a nightmare to open. You see it all in our lunchroom.
Also, teachers do not allow children, especially K, to opt out of eating lunch from home or ditching lunch from home to go through the line.
Are there no parent volunteers or teachers supervising your K group at lunch?