Then you’ve never tried to keep three growing children fed. And milk won’t last a month in the fridge.
Guess you haven’t lived in the country far from a grocery store or as Moo likes to say, food deserts.
Kids get milk at school for breakfast and lunch. How much needed water are your kids getting? There are other sources for calcium such as yogurt/yogurt drinks which lasts all month. All green leafy veggies are high in calcium as well as squash, peas, beets, tomatoes, beans, broccoli, bell peppers, potatoes and sweet potatoes. Hope you’re not pouring milk on sugary cereal for their breakfasts.
Two words - pantry and freezer.