Arti Dhir, 55, and Kaval Raijada, 30, from west London, deny arranging to have 11-year-old Gopal Sejani killed for an insurance pay-out in 2017.
Britain has so far rejected requests to extradite the couple to face trial in India on human rights grounds.
However, the Indian government has been granted leave to appeal the decision.
The husband and wife, from Hanwell, had travelled to Keshod, a town in Gujarat, to adopt an orphan in 2015.
According to court documents, Indian authorities say the couple placed an advert in a local newspaper, promising they would take an adopted child to live in London.
The couple then met Gopal, a farm boy who was living with his older sister and her husband, Harsukh Kardani.
The pair, who were his guardians, agreed to the adoption, believing the child would have a better life in the UK. They began preparing adoption papers.
However, Indian police claim Ms Dhir and Mr Raijada – who had no children of their own – had other plans.
Authorities in India say Ms Dhir took out an insurance policy in Gopal’s name. The policy was worth approximately £150,000 and would pay out after 10 years, or in the event of his death.
According to the documents, she made two premium payments, each of £15,000.
“After a few days she took out an insurance policy in his name,” Superintendent Saurab Singh of Junagadh Police, in Gujarat, told the BBC.
“It was a huge amount and she paid two premiums, knowing very well that in the event of Gopal’s death, she would be paid 10 times the insured amount.”
The couple returned home to London but Gopal never made it to the UK. He remained in Gujarat while visa papers were arranged for him.
On 8 February 2017, he was abducted by two men on motorbikes, stabbed and left by a road in Gujarat.
His brother-in-law, Mr Kardani, was also attacked as he tried to defend the boy. Both died of their injuries in hospital later that month.
Indian authorities say two previous attempts had been made against the boy’s life, but both failed. The insurance policy never paid out.
Officers in India arrested a suspect who they said was a friend of the couple and had spent time with them as a student in London.
He is one of four men who have been arrested in India for alleged involvement in the crime. The investigations are ongoing.
Ms Dhir and Mr Raijada, who face six charges in India, including conspiracy to murder and kidnapping, were arrested in the UK in June 2017 after a request from the Indian government.
However, on 2 July this year, a judge at Westminster Magistrates’ Court refused their extradition on human rights grounds.
In her judgment, Senior District Judge Emma Arbuthnot, found there was sufficient evidence to justify their extradition as there was a “circumstantial prima facie case that Ms Dhir and Mr Raijada acting together and with others committed the offences”.
But because the penalty for double murder in Gujarat is life in prison without parole, she ruled extradition would have been contrary to the couple’s human rights under UK law.
She said if extradited, the pair could be given “an irreducible sentence” and a lack of a review would be “inhuman and degrading”.
The Indian authorities have been granted an appeal, which is expected to be heard in the new year.
Commenting on the case, Nick Vamos, former head of the Crown Prosecution Service’s extradition team, said the decision had been made because there was no prospect of release, even in exceptional compassionate circumstances.
Outside the couple’s west London home, the BBC tried to question Ms Dhir about the case and why she was refusing to travel to India to stand trial. She refused to respond.
Both Ms Dhir and Mr Raijada deny the allegations. According to the court papers, they say there is “no prima facie case against them”.
The couple remain on bail pending an appeal.
Superintendent Singh added: “We are trying our best. This is a very serious offence that has taken place in India.
“We want the two accused to be brought here to face trial in an Indian court as per the Indian laws, and for this we are trying our best to assist the UK court.”
If the appeal fails, the chief magistrate said it was “not impossible” the couple could be prosecuted in the UK, if there was evidence that an agreement to murder was made in this country.