How do I explain breastfeeding my newborn to my older child?

I didn't explain it to my toddler, whom I bottle-fed my breastmilk to so she didn't know the concept of breastfeeding. So when her baby sister arrived, I just started doing it in front of her, and then I said, "I'm giving your sister her milk!" And she understood the word milk, and that whenever her sister cried, she normally was asking for milk. My toddler became my "milk nazi," in fact. Whenever her sister cried, she'd go, "Naomi needs milk, Mommy!!!!"