It's not that simple:

"Children play a part, too. A recent working paper published by NBER finds that children increase the happiness of European parents, especially for the first ten years. Parents become less happy, however, if having children causes them to encounter financial difficulties. It follows, then, that happiness bounces back for financially-stretched parents once those children become financially independent, but decreases for those parents that enjoyed their children in financial comfort."

So...

Children = Happiness - Money

...I knew it.

I recall Daniel Kahneman concluded the opposite: happiness is largely a function of spending time with people you love (not including children), and life-satisfaction is largely a function of plain old career success.

yeah, not quite. children actually tend to make you less happy than your peers. at least until they’re old enough to largely fend for themselves. At least in western cultures.

I suspect households with more than 2 adults the equation changes significantly. My experience is every child is a pleasure for 2 hours, no matter how badly they are misbehaving. So a household with enough adults to hand off the children may be able to maximize some kind of child additive happiness, though now you have to deal with added adult drama...