Luke got into his mother’s makeup and painted his face – and the walls. He toppled his father’s wine a few days earlier, overturned a plate of hors d’oeuvres, and made mud pies with the dog’s food.

The 15-month-old is a terror, his parents confess, but what can they do? He is still too young to discipline, isn’t he?

Not at all. This 15-month-old is very much ready for discipline, and the sooner, ...