Police said the incident happened on New Year’s Eve.
The two minors were inside their house together with their mother when an alleged stranger knocked on their door, police said.
And when the mother responded to the knock, a man armed with a knife burst into the room.
The man, according to the police, tied the woman onto a chair before turning to the children and killing them.
He then allegedly placed their bodies in two separate buckets full of water before escaping.
The woman did not raise alarm and it is not clear why.
It was until after a neighbor visited the house hours later that the incident was realized, police said.
Police are investigating the murder and are yet to establish the motive.
Police said the neighbors did not hear any screams from the house.
“We don’t know how and why this happened,” said Kikuyu police boss Catherine Ringera.
She added the woman was detained at the local police station as investigators work on the case.
The bodies were moved to the mortuary pending autopsy and probe.