According to this study, after adjusting for income, having children is actually associated with higher happiness and well-being.

From a Psychology Today article that summarizes it:

However, household income may not be a good indicator of financial stress. A family with low income that lives in an area with a low cost of living might experience less financial stress than a family with a higher income that lives in an area with a much higher cost of living. Therefore, the researchers conducted an additional analysis in which they included a direct measure of whether or not the family experienced difficulties in paying bills in the last year. This analysis showed that difficulties in paying bills represented a central influence factor for the relation of having children and parental well-being. When the researchers statistically controlled for financial difficulties, having children was actually related to greater well-being in parents.