Influencer Shay Mitchell launched a skincare line for kids called “Rini.” The backlash was fast and swift, with people debating whether kids need such extensive skincare.

My 11-year-old daughter is not on social media. She doesn’t even own a smartphone, and she has limits on screen time. But somehow, she still knows about the latest viral cosmetic trends on TikTok.

At least once a month, someone asks Kathryn Keough, a clinical psychologist at the mental-health nonprofit Child Mind Institute, if their child is in trouble. An expert on juvenile trauma and distress ...