Any income etc based coupon system is inefficient and automatically excludes a big portion of children that such measures are supposed to be for, eg because a lot of them come from families that are too dysfunctional to apply for those, ignorant of them due to language and other barriers, or because of (perceived or not) social stigma. And while adults are considered responsible for their own lives, it is a total moral bankruptcy for a society to have their children starve for their parents dysfunction. At the same time, providing free lunch to children at school solves/eases a lot of social, health and other issues all at once, for a cost that is basically peanuts compared to how impactful it can be.