Teach the kids that this planet cannot tolerate even one more human being now. We are already eight billion. At the current rate, we are projected to be eleven billion by 2050. We cannot even tolerate eight point one billion.
You do not merely want to have kids; you also want the kids to live happily, right? And happiness, in common parlance, means consumption. Even at the current rate of consumption, this entire planet is not sufficient to meet the needs of this population; you require multiple Earths. And this is at just the current rate of consumption, not the aspired rate of consumption. We all aspire to consume more and more, right? At the aspired rate of consumption, you will require a hundred Earths to fulfil the needs of these billions. From where will you get those hundred Earths?