Hanratty, Kian's Dad, took Valium that night and passed out. Prosecutors say he knew McMillan was high and should've ensured Kian's safety.
McMillan later tested positive for heroin, valium and alcohol. Jurors hear she was incapable of care and showed "wilful indifference".
The prosecution say Kian's death was "not a tragic accident" but "an entirely preventable death".