The incident took place last evening when one Ibrahim along with his grandchildren, including minor boy Mohammed Zaid, visited the bridge over Gola river, they said.
When he went to park his scooter, the woman lifted the boy and threw him into the river, police said.
Eyewitnesses soon took him out of the river and rushed to a nearby hospital where the boy was declared brought dead.
Angry locals caught hold of the woman and beat her up following which she was rescued by the police.
Further investigation is on, police said.