First, they would make a request for the person to pay. Then the party should be very diligent in making that request for not even just child support, but also orthodontic costs, medical costs, or extracurricular costs. The person seeking reimbursement should be diligent and not let months and months languish. If after making diligent requests the party is unsuccessful, they should hire an attorney to file a petition for enforcement.
Child support does earn 9% of interest per year. If someone is not paying child support for a long period of time, the arrears will accrue 9% interest, which definitely accumulates. The judges are required to respect that as it's in the statute.