Salt poison couple's letter plea
In letters to Radio 4's Today programme Ian and Angela Gay said they were "stunned" they had been convicted of the manslaughter of Christian Blewitt.
The three-year-old died in Birmingham in 2002, after falling unconscious at the couple's home in Bromsgrove.
The Gays, each jailed for five years in January, have launched an appeal.
Writing from prison, Angela Gay wrote: "For a brief time he was our son...and we loved him."
'Nightmare'Christian was put on a trial placement with the couple, together with his younger brother and sister, in November 2002.
Prosecutors said the Gays force fed the boy up to four teaspoons of salt, as a punishment for bad behaviour.
Tests found he had excessive levels of sodium in his blood and CT scans showed he had suffered brain damage.
The court heard evidence from numerous medical experts but was told it may never be known what happened to the boy.
In her letter, Angela Gay said: "For more than two years now Ian and myself have been living a nightmare. All we ever wanted was a family of our own, and yet we find ourselves in prison, the victims of a gross miscarriage of justice."
Home Office pathologist Peter Acland - who carried out the post-mortem tests on Christian - has already spoken of his doubts over the conviction.
He told Today deliberate poisoning was only one possible explanation for the death.
The salt could have been taken by accident, or the boy could have been suffering from a "disease process" no one had spotted.
'No evidence'
"I've not heard evidence that convinces me that this child has been deliberately poisoned by the defendants," he said.
He said the Gays had been forced to try to prove they did not poison the boy, which was "the wrong way" to assess such cases.
Ian Gay said he first heard of the salt poisoning accusation when the trial was already under way.
"It was only during cross examination that we were directly accused of force-feeding Christian salt. The prosecution had no evidence other than their imagination, that salt was force fed at all," he wrote.
Christian's natural family still believe the couple to be guilty.
The child's grandfather, Jim Burke, said the family were not told of any particular medical problems suffered by the toddler.
"How come this has only come out now? If it was right, surely we would have heard about it before now and the Gays would have used it in court. It just doesn't ring true."
Christian's natural grandmother, Susan Osik, said she still believed the Gays had killed him.
"They shouldn't get away with it," she told Today.
"They shouldn't do that to a child. If I had done that to a child, I would have got life."
The Gays' families are supporting their appeal.
Angela Gay's brother Carl Swain said they had often left their children with the couple, who "loved" being with them.
Ian Gay said they had been warned the jury may find them guilty of manslaughter, "because juries could behave in that way when the death of a young child is involved".
The couple's new legal team includes lawyers involved in the cases of Sally Clark and Angela Cannings - both of whom were wrongly convicted of killing their children.
"We now have to prove our innocence. We will not rest until we have done that, nor will our family," added Ian Gay.