When the going gets tough, the tough … pop in a pacifier? That’s the surprising stress-busting move thousands of young adults in China are making — swapping melatonin and meditation for an adult-sized binky. Once a niche quirk, the silicone soothers are supposedly now big business on Chinese e-commerce giants like Taobao and JD.com, where they are priced anywhere from a budget-friendly 10 yuan ($1.40) to a luxury 500 yuan ($70). They’re sold as sleep aids, stress relievers, even smoking-cessation tools — and shoppers are gobbling them up. Some online sellers move thousands each month, according to the South China...