"Number of kids you have" is a strange place to focus on environmental impact, don't you think? A modest household with 6 kids, even one that lives to developed-world standards, has much less of an environmental impact than a single billionaire with a private jet. Like, orders of magnitudes less. If the family has one car and doesn't eat a lot of beef they probably have less of an impact than a family with 2 kids and 2 cars that goes to McDonald's a few times a week.

Basically, the environmental impact of having more kids is sort of drowned out by various consumer choices, which are in turn drowned out by societal choices that no one family can impact at all.