If you look at the data - the same GSS data you favorably cite - you'll see that kids usually have a smaller negative effect on the happiness of moms than the happiness of dads. The natural inference is that you're missing half the story. Yes, women bear more of the costs of kids, but apparently they also get more of the benefits.
...I noticed that my original result was fragile. Controlling for real income, church attendance, age, and marital status, men take a bigger happiness hit from kids than women, just as I said. On a 1-3 scale, every child predicts a 0.021 reduction in male happiness, but only a 0.016 reduction in female happiness.
However, I later noticed that controling for gender reverses the result, because all else equal, the average women is a little happier than the average man. Adding a gender control to my previous specification, each child reduces male happiness by .014, and female happiness by .022. That's still really small - by way of comparison, married people have score about .19 units higher than singles in both specifications. But qualitatively, the data support Will's claim that on balance, being a parent is harder on women than men.
Of course, as I've argued before, the wise solution to gender conflict is Coasean bargaining, not resentment.