A jury in Nottingham Crown Court deliberated for only four hours before finding Sally Dent, 33, and Shaun Binfield, 45, guilty, The Guardian reported. Dent was also convicted of cruelty to a child.
The couple are scheduled to be sentenced Feb. 19.
Riley died in March. The couple admitted during the two-week trial that a child's cup containing methadone had been left where he could reach it, saying they usually put it on top of a bureau.
The methadone had been prescribed for Dent. Witnesses testified she sometimes sold her supply to make money.
Binfield was almost crying when he testified about failing to move the cup. He said he was distracted by putting the trash out and getting his children up.
"If the alarm bells had rung I would have removed it and my son would still be here," he said.