The incident took place yesterday when the child, identified as Amrit was playing near the septic tank, police said.
"Amrit was playing near the tank which was left open for getting it cleaned. The labourers were also not around at the time the incident took place. All of a sudden the child came near the tank, lost his balance and slipped into it and drowned. He was rushed to Safdarjung hospital and was declared brought dead," said a senior police official.
Police have registered a case of 304A (causing death by negligence) against the landlord and the labourers who had left the tank open.
The body of the child has been sent for post mortem, they added.