In a child support case, the plaintiff is the person who files the petition asking a court to order support — most often the custodial parent, but sometimes a legal guardian or non-parent caregiver who has custody of the child, and sometimes the state itself when the custodial parent receives public assistance. The other parent becomes the defendant. That designation sticks for the life of the case and determines who presents evidence, who the court looks to for information about the child’s needs, and who ultimately receives the payments.
The Custodial Parent Is Usually the Plaintiff
The parent with primary physical custody is the one who almost always starts the case. Filing means submitting a petition or complaint to your local family court. Whoever files first is the plaintiff; the other parent is the defendant. That label carries through the life of the case, including any later proceedings.
As plaintiff, you carry the burden of showing the court two things: what it costs to raise your child, and what the other parent can afford to pay. That means gathering documentation of the child’s expenses alongside evidence of the non-custodial parent’s finances. The court then applies your state’s child support guidelines to arrive at a monthly amount.
Filing fees for a new case range from nothing to roughly $400 depending on the jurisdiction. Many courts waive the fee entirely when you go through the state child support agency, and fee waivers for low-income filers are available in most places.
When a Non-Parent Can File
Custodial parents aren’t the only people who can bring a child support action. Grandparents, other relatives, and non-parent caregivers who have legal custody or court-appointed guardianship of a child can file as the plaintiff. The key word is “legal.” Informally caring for a child, even for years, doesn’t automatically give you standing. You need a court order establishing you as the child’s legal custodian or guardian before most courts will let you seek support from the child’s parents.
When evaluating whether to grant that standing, courts look at who has been providing daily care, how long the arrangement has existed, whether the parents have stayed involved or contributed financially, and whether remaining with the guardian serves the child’s best interests. If you’re a relative raising someone else’s child without a custody order, getting that legal recognition in place first is a practical prerequisite to filing for support.
When the State Is the Plaintiff
If you receive Temporary Assistance for Needy Families (TANF) benefits, the state can step into the plaintiff role and pursue child support on its own. The legal foundation is Title IV-D of the Social Security Act, which funds and requires every state to operate a child support enforcement program.1Social Security Administration. Social Security Act 451 – Appropriation
Assignment of Rights Under TANF
Here’s the part that catches people off guard: when you sign up for TANF, you automatically assign your right to receive child support payments to the state. The state collects support from the non-custodial parent and keeps enough to reimburse itself for the benefits it paid you. You don’t see those payments while you’re on TANF, except for a small pass-through amount that varies by state. Once you’re off public assistance, the assignment ends and support payments flow to you again.
What the State Agency Does — and Doesn’t
State child support agencies handle much of the heavy lifting, and their services aren’t limited to TANF recipients. Any custodial parent can apply for help through the Title IV-D program. The agency will locate the other parent, help establish paternity if needed, file for a support order, and enforce it going forward.2Social Security Administration. Social Security Act 454 – State Plan for Child and Spousal Support These services are either free or very low cost.
One important limit on the agency’s role: it represents the interest of the child, not you personally. The agency won’t handle custody disputes, divorce proceedings, restraining orders, or spousal support. If your case involves any of those alongside child support, you’ll need your own attorney for everything except the support piece.
Paternity Has to Be Established First
Before any court will order child support, there has to be a legal parent-child relationship on the books. For married couples this is straightforward, because the spouse is presumed to be the other parent. For unmarried parents, paternity must be established before a support case can move forward.
There are two main paths. The simpler one is a voluntary acknowledgment of paternity, signed by both parents and authenticated by a notary or witnesses. Hospitals routinely offer this form at birth. The second path is genetic testing ordered through the child support agency or the court. If either parent in a contested case requests testing, the agency is required to arrange it.3eCFR. 45 CFR 303.5 – Establishment of Paternity The agency pays for the initial test, though it can recoup the cost from a father who denied paternity and was proven wrong.
If the alleged parent ignores the case after being properly served, the court can enter a default paternity order. Once paternity is legally established, the support case proceeds.
What the Plaintiff Needs to Bring
Whether you’re filing on your own or through a state agency, arriving prepared makes a real difference in how quickly your case moves. The federal Office of Child Support Services recommends bringing the following to your initial appointment:4Administration for Children and Families. What Documents Do I Need to Bring to the Child Support Office
- Information about the child, including birth certificates, Social Security numbers, and any existing custody or support orders
- Your own financial records: pay stubs, tax returns, and documentation of income and assets
- Whatever you have on the other parent’s finances — income, employer, bank accounts, investments, or property
- Records of child-related expenses such as healthcare, daycare, and any special needs costs
- Identifying details for the other parent: address, employer name and address, date of birth, Social Security number, and a recent photograph if available
You won’t always have everything on this list, especially the other parent’s financial details. Bring what you can. The child support agency has tools to fill in the gaps, including the Federal Parent Locator Service, which cross-references federal databases to track down a parent’s address and employer.5Administration for Children and Families. A Guide to the National Directory of New Hires
The Plaintiff Role in a Modification
The original plaintiff and defendant labels don’t lock the parties into those positions forever when it comes to changing the order. Either parent can file to modify the amount if circumstances change significantly, and whoever files the modification becomes the plaintiff in that proceeding. Courts look for a material change in circumstances before they’ll adjust the order. Common grounds include:
- Major income changes, such as a job loss, a significant pay cut, or a substantial raise for either parent
- Shifts in custody or parenting time that change how costs are shared
- Significant increases in healthcare, childcare, or special needs expenses
- A child aging out of the order, which should trigger a recalculation for any remaining children
Modifications take effect from the date you file the request, not retroactively. If your income dropped six months ago but you waited to file, you’re still on the hook for the original amount during those six months. Filing promptly when circumstances change protects you from accumulating arrears you can’t afford.