When our second child was on the way, my wife and I attended a pre-birth class aimed at couples who had already had at least one child.
The instructor raised the issue of breaking the news to the older child.
It went like this:
"Some parents," she said, "tell the older child, 'We love you so much we decided to bring another child into this family.' But think about that. Ladies, what if your husband came home one day and said, 'Honey, I love you so much I decided to bring home another wife.'"
One of the women spoke up immediately. "Does she cook???"