"Welcome! Play Safe," reads the sign at a Fairfax County Public School playground in Virginia, just outside of Washington, D.C. The sign also lists a few simple rules—21 of them, by my count. First ...

You — and your child — are probably making a mistake on the playground slide. “My biggest tip for slide safety is to not go down it with your child,” Olivia Reyes, a developmental physical therapist, ...