Updated: Foster pair of boy, four, claimed harassment by Edlington attack brothers
Marcia Langton, 46, was arrested last night after police launched a hunt for her and husband Darren Langton, 40, and the child, named Kanan.
But the couple did not turn up to hand him over to social workers, sparking a police appeal to find them.
Lincolnshire Police said Kanan was found by police in Doncaster last night, who informed them at about 7.15pm.
A spokeswoman said 46-year-old Mrs Langton was arrested and taken back to Skegness police station, while her husband was still being sought.
She was due to be interviewed today by detectives investigating the case, the spokeswoman said.
Officers were still looking for Mr Langton today, while Kanan was returned to Newham Children's Services.
Yesterday it emerged the couple were facing court action over the four-year-old's return to local authorities.
Newham Council said "serious concerns" prompted it to take court action to return him to their care, leading to the couple's disappearance.
A spokeswoman said: "Kanan was placed with the couple 16 months ago. They were being considered as suitable adopters.
"In recent weeks, we were made aware of serious concerns which led us to take court action to return him to our care.
"This action by the council led to the couple's disappearance and we alerted the police straight away."
Today a spokeswoman for the council said they were "delighted" he had been found.
Detective Inspector Andy Wardell, from Lincolnshire Police, said: "Following extensive and rapid inquiries carried out in partnership with our colleagues in South Yorkshire Police, a 46-year-old woman is now in custody at Skegness Police Station."
The Yorkshire Post has seen a copy of an email sent to Doncaster council by Mrs Langton, a 46-year-old black woman, which is an apparent plea for help and includes claims the family "suffered a horrendous catalogue of racism".
At one point it specifically pinpoints the alleged role of the two brothers who went on to commit the Edlington attacks.
The council, the subject of severe criticism for failures leading up to the Edlington attacks, declined to comment on the allegations last night.
Chris Pratt, director of children and young people's services, said: "These are confidential matters and therefore we do not comment on individual cases. However, we take all allegations of anti-social behaviour and racial abuse very seriously and investigate them thoroughly."
A complicated picture of the Langtons' life in Doncaster emerged yesterday, including Mr Langton's conviction for an assault on a neighbour. The 40-year-old, who is white, was fined 200 by Doncaster magistrates on August 25, ordered to pay 100 compensation and placed under a curfew.
The Langtons moved with four-year-old Kanan, who is under the care of Newham Council in London, to Skegness within the past six weeks. He was put in their care 16 months ago and the council was considering them as potential adopters.