That's the conclusion of recent research cited by the Washington Post. Here is one of the key paragraphs:
For men, fathers of one child and those without children similarly throughout much of their careers. But men with two or more kids were more productive than both groups. The effect for women was even more dramatic. Using their own method for analyzing research publications, the authors found that within the first five or so years of their career, women who never have children substantially underperform those who do. (The difference in productivity between women with one child and those with no children is more muted using a different ranking for research. But in both cases, mothers with at least two children perform the best.)
One of the reasons given by the Washington Post for such surprising data is that mums with multiple children tend to be hyper-organized. Whilst agreeing with this, pastor-theologian Peter Leithart adds another important factor - the effect parenting has on settling people:
The research may be “counterintuitive,” but anyone with kids knows it’s accurate. Like marriage, parenthood focuses attention and forces efficiency in time management. Probably more importantly, parenthood, like marriage, settles, and no one is less productive than the unsettled.
That intiuitively makes sense to me and as we are expecting the third kipling I'm off to share this with the lovely Mrs K.