Enough already. If there’s nothing innately unhealthy about any individual child’s following a parent into politics, we still should look askance when the practice becomes habitual, and when the habit grows so common. The always-commonsensical Barbara Bush, when famously recommending against a Jeb Bush candidacy a few months ago by saying “we’ve had enough Bushes,” was making a broader point. “I think it’s a great country,” she said. “There are a lot of great families. It’s not just four families or whatever. There are other people out there that are very qualified.” In a nation of more than 300 million...