Some studies had found that having children did not clearly increase parents' happiness, while others found that it did. Luis Angeles has cleared up the mystery, I think: Kids increase the happiness of married parents, but decrease it for single parents and cohabiting couples.
Moreover, Angeles found that married parents' happiness increased with each child up to three kids, which he thinks is the amount of kids most married couples want to have. Married mothers were made happier by having children than married fathers were - with three kids, mothers were made about twice as happy as fathers.
[ Luis Angeles, “Children and Life Satisfaction”, Journal of Happiness Studies, 2009]