According to BBC, Samantha Pendo had been in a coma since Friday after sustaining head injuries when police stormed homes in a slum in western Kisumu city.
Her father, Mr Joseph Abanja, told Kenya's privately owned Daily Nation newspaper: "I demand justice for my daughter. She was not protesting, her father was not protesting. Her mother was not protesting. Yet they killed her.
I did as I had been told; to vote and go home. They followed us there and hit us."
At least 24 people were killed in violence which broke out after the 8 August election, according to a Kenyan human rights group.
Opposition leader Raila Odinga said the poll was rigged to give President Uhuru Kenyatta victory.
However foreign observers said the poll was credible.