The toddler wriggled across the exam table, laughing as he crinkled its paper covering. His grandmother JoAnne scooped him onto her lap, rubbing his back as he sucked his thumb. She lifted his hospital gown and unfastened his diaper, showing us the raw skin beneath. Teary-eyed, she told me and another pediatrician the story of his rash. About a year earlier, JoAnne had received full custody of 2-year-old Calvin when his parents, struggling with drug addiction, could no longer adequately care …