Child support is intended to cover the expenses of the child - food, clothing, education, medical, etc. It's not meant to pay for the time spent by the custodial parent in actually doing the parenting, nor any sort of "opportunity cost" of not being otherwise engaged in a paying job.

Translated to HN-world: "I'm going work on a FOSS project full-time!" World: "Great; we'll pay for your server! You won't lose a dime!" Me: "Uh, what about the income from the job I gave up?"