Daniel Gilbert, the Harvard "Professor Happiness" presented research at a conference in Sydney on Happiness and Its Causes. He reported that marriage makes people happier, which I have often noted here. He also reported some detailed research on the relationship between having kids and happiness.
Parents are happiest when they are anticipating having kids, and after the kids are grown. Parents are least happy, in general, when they have adolescents at home.
One of the useful warnings I can give students in my family class is that they need to have a strong marriage before they have children so that they can weather the "parental crisis" that coping with a baby brings. It does not surprise me that self-reported happiness for married couples goes down with the birth of children, and troughs in the adolescent years.
I think Gilbert errs, though, in how to weigh the whole cycle of parental happiness. He thinks expectant parents overestimate how happy children will make them. Reality sets in when actual demanding children take up your time. Then empty nesters overvalue how much happiness they got from parenting in order to retroactively justify the investment.
I think happiness is the wrong metric here. When we ask parents what is the most worthwhile or meaningful thing they have done in their lives, raising children consistently rates at the top. I think people rate happiness on a more self-oriented scale. Not necessarily selfish; but measured against how it makes me feel now. Meaningfulness, on the other hand, is measured against a broader standard. We are often willing to sacrifice short-term feelings of happiness for long-term value, good, meaning.
The same surveys that ask "Do you feel Very Happy, Somewhat Happy, or Not Too Happy" should add another question. They should ask something like "Do you think your life is Very Meaningful, Somewhat Meaningful, or Not Too Meaningful."
On that scale, I think parents would give higher ratings.