"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.
FWIW, "number of flights you take" also drowns out your eating habits in environmental impact. Compared to how much they cost, flights have stupid CO2 equivalents.
However, I don't know why you are comparing a single billionaire vs a single X kid household. Like, the number of each (or even of private jets) are not even _remotely_ in the same ballpark. Which is why "number of kids" is not at all a strange place to focus on environmental impact, but "billionaire lifestyle choices" is.
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.