Dana is exactly right that the state garnishing wages for child support and withholding that money is bad public policy, but we should stop mincing words about what is actually happening. The state is taxing child support. What's worse is that witholding that money interferes with families going off public assistance, keeping them dependent on taxpayer money longer than it would if the state simply passed the child support on to the family.
Garnishing wages in this manner doesn't exactly "discourage work," it discourages work for wages that aren't paid in cash. It encourages non-custodial parents to enter the underground economy, which can be as innocuous as construction or as dangerous as drug dealing. This isn't to remove responsibility from people who make bad decisions, but it is to say that public policy can discourage some behaviors and encourage others, and we should be aware of which policies do facilitate bad behavior and which do the opposite.
Taxing child support hurts families and encourages bad behavior. Period.
--A. Serwer