Watch your baseline: you should not consider the benefits that you and your child might get vs. not having children, but rather, the benefits you and the child might get vs. the benefits that you and another child might get if you did not have a child but became involved in a mentor program (or other volunteer activity helping children).

It may be hard to determine the value you get through working with other people's children, but there are big two plus sides to doing so:

1. you have a comparative advantage for a certain population of kids; those with mental illnesses may benefit especially from an adult who has experienced something like what they are going through and

2. you can experiment to determine the value you get from a mentor program much more easily (or rather, with much lower cost) than you can experiment with having your own kids -- and it makes good sense to try the low cost experiment before you run any final calculations.