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".
More top news
Cold and blustery with clear spells and wintry showers. Risk of ice and snow cover over hills by dawn on Monday
Windy. Wintry showers, giving snowfall over hills and ice risk.
Showers turning increasingly wintry after dusk. Snow may settle over higher routes by dawn. Risk of ice for the Monday rush hour