David Carroll Jr. and Amy Baker face a felony charge of tampering with physical evidence, which stems from the dumping of the boy's body in the Ohio River, Maysville police said in a statement.
Carroll and his wife, Liz, left Marcus Fiesel wrapped in blanket and packing tape at their home in Batavia, Ohio, while they went to family reunion in Kentucky.
David Carroll, 29, was sentenced in February in Clermont County, Ohio, to 15 years to life in prison for the developmentally disabled boy's death. Liz Carroll was sentenced to 54 years to life in prison for murder and other charges.
Baker, who lived with the Carrolls, was given immunity in Ohio to testify against the Carrolls about Fiesel's death. She testified at Liz Carroll's trial that she was with David Carroll when Fiesel's remains were thrown from a bridge over the Ohio River.
Maysville police filed a complaint alleging that David Carroll and Baker threw Fiesel's burned body into the Ohio River from a bridge in Mason County in northern Kentucky on Aug. 7. Police said the pair intended to destroy or conceal evidence in Fiesel's death.
David Carroll is serving his sentence in a state prison in Lucasville, Ohio. Baker was free Friday on $50,000 bond after being booked into the Mason County jail.
Other than the written statement confirming the charges, police and prosecutors in Kentucky declined to comment Friday.
Clermont County Prosecutor Don White could not immediately be reached for comment. A message seeking comment was left Friday for Cathy Adams, David Carroll's lawyer.
Mason County Attorney John Estill previously told the Ledger-Independent in Maysville that he promised Ohio prosecutors he wouldn't interfere with the murder investigation, but after both Carrolls were convicted, "the issue is moot."
Baker acknowledged her participation in burning the child's body, but she was not charged in exchange for her testimony against Liz Carroll, who was convicted of murder, involuntary manslaughter, kidnapping, felonious assault and three charges of child endangering. Ohio prosecutors said she would be prosecuted if proof surfaced that she had hands-on involvement in the boy's death.
Baker testified at Liz Carroll's trial that she and David Carroll drove across the bridge at Maysville into Kentucky, then turned around before dumping the remains in the river.
Prosecutors say the couple found Fiesel dead when they returned from the reunion to their home east of Cincinnati and about an hour's drive from Maysville. The Carrolls then claimed the child had disappeared from a park in suburban Cincinnati, sparking a search by thousands of volunteers.
The case sparked calls for changes in the way Ohio dealt with foster children. It also prompted the state to revoke the license of Lifeway for Youth, the private agency that was handling Fiesel's case.