It’s vital that a helmet fits as securely as it can. “Kids’ bike helmets are not one-size-fits-all,” says Natalie Martins, who runs the popular kids’ bike blog Twowheelingtots.

If you could go back 50 years, you would see very few kids wearing helmets as they zoomed around on bikes. Who knows if, 50 years from now, bareheaded kids on the playground circa 2018 might look ...