A dad complained that he hasn't had contact with his children in years, and mom keeps moving to different states, mom has blocked all phones so he can't contact his children, and the child support agency claims they cannot help him. He has gone to court many times with no results.

I am not a attorney and this is not legal advice.

Your state has a parent locator service.

45 CFR § 302.35 - State parent locator service.

(1) For IV-D cases and IV-D purposes by the IV-D agency. The State PLS shall access the Federal PLS and all relevant sources of information and records available in the State, and in other States as appropriate, for locating custodial parents, noncustodial parents, and children for IV-D purposes.

https://www.law.cornell.edu/cfr/text/45/302.35

Title IV-D includes parenting time enforcement. This includes:

42 U.S. Code § 669b - Grants to States for access and visitation programs

https://www.law.cornell.edu/uscode/text/42/669b

If someone deceptively misrepresents that the locator service can't be used to find children for court ordered parenting time, that may be fraud.

A motion for contempt of court is one path to seek parenting time:

Sometimes a former, or soon-to-be former, spouse or co-parent can make your life difficult by refusing to pay support or cooperate with parenting time or fighting your efforts to resolve your divorce. When inconvenience and annoyance crosses the line into the violation of court orders, you may be able to use contempt of court proceedings to correct the other party’s behavior and get your case, and your life, back on track.

https://nsssb.com/contempt-of-court-in-michigan-family-law-cases/

If someone deceptively misrepresents that the other parent can't be held in contempt because they cannot be served and the locator service cannot be used, this might be:

Obstruction of justice Definition 18 U.S.C. § 1503 defines "obstruction of justice" as an act that "corruptly or by threats or force, or by any threatening letter or communication, influences, obstructs, or impedes, or endeavors to influence, obstruct, or impede, the due administration of justice."

https://www.law.cornell.edu/wex/obstruction_of_justice

It also might be:

Fraud

Fraud is both a civil tort and criminal wrong.

In civil litigation, allegations of fraud might be based on a misrepresentation of fact that was either intentional or negligent. For a statement to be an intentional misrepresentation, the person who made it must either have known the statement was false or been reckless as to its truth. The speaker must have also intended that the person to whom the statement was made would rely on it. The hearer must then have reasonably relied on the promise and also been harmed because of that reliance.

https://www.law.cornell.edu/wex/fraud

If there is a concern that the other parent may be endangered, reasonable efforts can be made to utilize a lawyer for that parent and a process server to provide notice.

Possible cause for a federal civil rights action under:

42 U.S. Code § 1983 - Civil action for deprivation of rights

https://www.law.cornell.edu/uscode/text/42/1983

US courts provide forms:

Complaint for Violation of Civil Rights (Non-Prisoner)

https://www.uscourts.gov/forms/pro-se-forms/complaint-violation-civil-rights-non-prisoner

18 U.S. Code § 242 - Deprivation of rights under color of law

https://www.law.cornell.edu/uscode/text/18/242

When one parent isolates the children they share from the other parent in violation of a court order, it may be domestic violence:

What Is Domestic Violence?

Does your partner ever….

Insult, demean or embarrass you with put-downs?

Stop you from seeing your friends or family members?

Make all of the decisions without your input or consideration of your needs?

Tell you that you’re a bad parent or threaten to take away your children?

https://www.thehotline.org/is-this-abuse/

All victims have equal access:

"No person in the United States shall, on the basis of actual or perceived ...sex..be excluded from participation in, be denied the benefits of, or be subjected to discrimination underany program or activity funded in whole or in part with funds made available under [VAWA]"

https://www.justice.gov/sites/default/files/ovw/legacy/2014/06/20/faqs-ngc-vawa.pdf

See also:

Improving Law Enforcement Response to Sexual Assault and Domestic Violence by Identifying and Preventing Gender Bias

https://www.justice.gov/ovw/policing-guidance

I would also consider a lawsuit for intentional infliction of emotional distress. This may also apply to any party assisting the other parent in their campaign of domestic violence, including a grandparent.

https://www.law.cornell.edu/wex/intentional_infliction_of_emotional_distress

Years without contact while a parent suffers sounds like depraved indifference.

To constitute depraved indifference, the defendant's conduct must be 'so wanton, so deficient in a moral sense of concern, so lacking in regard for the life or lives of others, and so blameworthy as to warrant the same criminal liability as that which the law imposes upon a person who intentionally causes a crime.

https://definitions.uslegal.com/d/depraved-indifference/

Denying a parent and child a healthy relationship may be child abuse. Long standing science shows that parents with fathers in their life have improved outcomes. The same is true of mothers.

Research shows that a loving and nurturing father improves outcomes for children, families, and communities. Fathers who live with their children are often more likely to have a close, enduring relationship with their children.

https://www.fatherhood.gov/for-dads/father-presence

If officials know of this abuse and fail to report it may be breaking the law.

A person who, while engaged in a professional capacity or activity described in subsection (b) of section 226 of the Victims of Child Abuse Act of 1990 on Federal land or in a federally operated (or contracted) facility, or a covered individual as described in subsection (a)(2) of such section 226 who, learns of facts that give reason to suspect that a child has suffered an incident of child abuse, as defined in subsection (c) of that section, and fails to make a timely report as required by subsection (a) of that section, shall be fined under this title or imprisoned not more than 1 year or both.

18 U.S. Code § 2258.Failure to report child abuse

https://www.law.cornell.edu/uscode/text/18/2258

State child support, parenting time, and child protection, and elder abuse are all primarily funded under federal law Title IV-D and Title IV-E. Consider filing complaints.

HHS Office of Inspector General

Phone. 1-800-HHS-TIPS (1-800-447-8477)

https://oig.hhs.gov/fraud/report-fraud/contact.asp

HHS Office of Civil Rights

https://www.hhs.gov/civil-rights/filing-a-complaint/index.html

Department of Justice Civil Rights

The Civil Rights Division of the Department of Justice, created in 1957 by the enactment of the Civil Rights Act of 1957, works to uphold the civil and constitutional rights of all Americans, particularly some of the most vulnerable members of our society. The Division enforces federal statutes prohibiting discrimination on the basis of race, color, sex, disability, religion, familial status and national origin.

https://www.justice.gov/crt

Anyone may report federal crimes to the FBI.

(202)324-3000

http://tips.fbi.gov/

I wish you the best of luck in your search for justice.