One evening, we asked our 2.5-year-old daughter a simple question: “How many people live in our house?” We expected her to say four—me, my husband, her, and her baby brother. Instead, she answered immediately: “Five.” We laughed, assuming she meant the cat. But she shook her head. “No. Mommy, Daddy, me, little brother… and…” She pointed toward the hallway. An...