In April 2019, police were called after the child stopped breathing.
23-year-old Naomi Johnson and 26-year-old Benjamin O’Shea were arrested, and police say they were “steadfast” in their refusal to answer questions.
The pair will be sentenced on Friday.
Detective Constable Caroline Baker says it’s “difficult to speak of justice in a case like this” but “children matter and the abuse they suffer will be punished”.