Michala Pyke and John Rytting allegedly encouraged Poppy Widdison to eat sedatives because they felt she was an inconvenience to their relationship, the trial at Hull Crown Court heard.
Poppy died in June 2013 after suffering a cardiac arrest at drug dealer Rytting’s “squalid” home in Grimsby.
Tests carried out after she died found that Poppy had ingested a variety of drugs for a period of up to six months before her death, including sedatives, heroin, methadone and ketamine.
Text messages between Pyke and Rytting talked about Poppy having a “blue Smartie” - believed to be a reference to the sedative diazepam, in the form of a blue tablet - and going to sleep.
David Gordon, prosecuting, told the jury earlier in the three-week trial: “We say Pyke and Rytting, the defendants, are just wanting to get on with their love life, wanting to enjoy each other’s company and it may be this young girl was something of an encumbrance.
“It’s apparent from the text messages that Miss Pyke viewed Poppy as an inconvenience, who she felt was in the way with regards her relationship with Mr Rytting.”
An ambulance was called to Rytting’s home in Oliver Court on June 9 2013, where Poppy was found to be “unresponsive, blue and not breathing”.
She was taken to hospital but died the next day.
A post-mortem examination could not establish a cause of death but toxicology tests carried out on her blood and hair found various drugs and showed the young girl had been exposed to and had ingested significant amounts of heroin and methadone for a period of between two and six months before her death.
The drugs did not contribute to Poppy’s death but experts agreed there was a “long period of ill-treatment and neglect by the grossly inappropriate administration of various drugs to the child by the defendants”.
The trial was told that Pyke was in the process of moving into Rytting’s home, where prescription and controlled drugs were “left lying around”.
Both defendants pleaded guilty to child cruelty by allowing Poppy to be accommodated in a house where prescribed and controlled drugs were unsecured and within reach of the child.
Pyke, 37, also pleaded guilty to child cruelty by emotional abuse after neighbours heard her shouting at Poppy.
On Friday, a jury took just over two hours to find both defendants guilty of one count of child cruelty by encouraging Poppy to ingest prescription and/or controlled drugs.
Rytting, 40, who admitted one count of importing drugs and two counts of supplying controlled drugs, was also convicted of possessing cannabis with intent to supply.
Pyke, who admitted a charge of supplying methadone, was found guilty of possessing methadone with intent to supply.
Recorder of Hull Judge Jeremy Richardson QC said: “You have been convicted by the jury of very serious crimes. You have, prior to today, pleaded guilty to very serious crimes.
“These crimes will inevitably attract a substantial prison sentence.”
The defendants showed no emotion as the jury returned the verdicts but shouting could be heard coming from the cells after they were remanded in custody and taken away from the court.
They will be sentenced next year.
A count of child cruelty by assault causing bruising against both defendants was dropped by the prosecution during the trial.