Some parents are choosing to forgo high-intensity camps and activities for their children in favor of weeks of unplanned time. Call it “kid rotting.”

Mothers have been exhausted since the beginning of time, but some difficulties are specific to the millennial generation.

Two decades ago, Rosalind Wiseman defined teen-girl dynamics and inspired Tina Fey’s cult classic movie. Now she helps the adult women who can’t escape high...