Back-to-school season is the perfect time to tune up healthy habits. Think of your child’s immune system like a team: sleep, nutrition, movement, and handwashing are the starters; smart supplementation can be the bench that helps you through the long season. Here’s how vitamins D and C, elderberry, and a probiotic can fit into a simple, safe routine—plus what to watch for. Vitamin D: the quiet powerhouse Vitamin D helps immune cells communicate and may reduce the risk or severity of some infections. Because kids spend much of the day indoors (and sunscreen, rightly, blocks UV), vitamin D is the supplement most children are likely to need. Vitamin C: best…