That is a really skewed stat. Having kids is only the worse thing a person can do, because of the average person's lifestyle (and therefore that kid's future lifestyle). How many cows are born each year (and release methane gasses into the atmosphere) just because the average person won't go without meat or milk?

If the average person didn't drive and was vegan, then the impact of having kids would be severely reduced.

EDIT: Put it this way, what do you think would be the environmental impact of a hermit dude living in the mountains who grows his own plant food? It might be an extreme example, but the point is illustrated.