It depends, the IRS confiscates refunds for back child support if your state enforcement is involved. If your state enforcement is not involved and she is getting a refund the IRS does not know that she owes you back child support and will not confiscate her refund to pay the back child support
As to who can the children there is a form 8332 to release dependent that is helpful to have in your records as to who can claim which children and what years.
Link to form 8332
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