Per IRS Publication 17 Your Federal Income Tax (For Individuals): 

Page 93: Child support payments. You shouldn't report these payments on your return. See chapter 18 for more information.

Page 134: Child support. A payment that is specifically designated as child support or treated as specifically designated as child support under your divorce or separation instrument isn't alimony. The amount of child support may vary over time. Child support payments aren't deductible by the payer and aren't taxable to the recipient.