What if I owe child support

I will assume this is in regard to the second stimulus check ... if so it cannot be taken for any debts including child support  BUT only if you get the stimulus payment in advance prior to 1/16 ... if you apply for the missing money  for  either stimulus credits on the 2020 return then all bets are off.   The credit is added to all your other payments and refundable credits  and if you have a refund on the return it can be taken ... see the line/form instructions here:  

From the 1040 instructions page 58:

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

Line 34 Amount Overpaid

If line 34 is under $1, we will send a refund only on written request.

Refund Offset

If you owe past-due federal tax, state income tax, state unemployment compensation debts, child support, spousal support, or certain federal nontax debts, such as student loans, all or part of the overpayment on line 34 may be used (offset) to pay the past-due amount.

Offsets for federal taxes are made by the IRS. All other offsets are made by the Treasury Department's Bureau of the Fiscal Service. For federal tax offsets, you will receive a notice from the IRS. For all other offsets, you will receive a notice from the Fiscal Service. To find out if you may have an offset or if you have any questions about it, contact the agency to which you owe the debt.

Injured Spouse

If you file a joint return and your spouse hasn’t paid past-due federal tax, state income tax, state unemployment compensation debts, child support, spousal support, or a federal nontax debt, such as a student loan, part or all of the overpayment on line 34 may be used (offset) to pay the past-due amount. But your part of the overpayment may be refunded to you if certain conditions apply and you complete Form 8379. For details, use Tax Topic 203 or see Form 8379.