PROVIDENCE — The Rhode Island Affiliate of the American Civil Liberties Union is the latest organization to throw its support behind the state child advocate’s legal fight to force change in the state’s care of foster children.

The ACLU Wednesday filed a “friend of the court” brief urging the 1st U.S. Circuit Court of Appeals in Boston to reinstate a lawsuit that charges mistreatment by the state Department of Children, Youth and Families of foster children in its custody.

Child Advocate Jametta O. Alston and the organization Children’s Rights brought suit against the DCYF in 2007, charging that the agency systemically violated the constitutional rights of children in state custody by exposing them to abuse and neglect. The suit, filed on behalf of 3,000 foster children, alleged that the child-welfare system was underfunded, understaffed and mismanaged, and asked the courts to intervene.

Senior U.S. District Judge Ronald R. Lagueux dismissed the suit in April. He ruled that Alston and the adults designated to represent the children did not have the authority to proceed because only guardians appointed by state Family Court, who declined to participate in the case, had standing to bring a federal lawsuit on their behalf. The judge found that the three people acting on the children’s behalf did not have close-enough relationships. Federal rules require an adult to stand in for any child who is a party in a lawsuit.

Alston and Children’s Rights appealed Lagueux’s decision to the First Circuit last week, arguing that his dismissal of the case deprived the children of their right to access the federal courts. The appeal alleged that the DCYF practice of shuffling children from home to home is responsible for their lacking a network of stable adults who could act on their behalf by the district court standards.

Fifteen children’s legal rights groups last week filed briefs backing that appeal. The ACLU joined them Wednesday, arguing that the judge’s decision violated a federal court rule designed to provide children with access to the courts. The ACLU asserts that federal court is the proper venue to hear cases challenging the constitutionality of state institutions.

“They need this court to open those courthouse doors for them. It is the just and proper result,” Andrew B. Prescott, the ACLU’s lawyer, wrote.

The state’s response to Alston’s appeal will come at the end of August, according to Kevin Aucoin, DCYF chief counsel.

The DCYF has made strides in the past two years that include reducing social workers’ caseloads; decreasing the number of youths in group homes or foster care; providing more community-based support services; and speeding the process for licensing foster homes, Aucoin said. The changes are intended to place children in the least restrictive environments, he said.

kmulvane@projo.com