Ms Sunita and her son were allegedly hit multiple times with a blunt object, the police said.
Last year, a 17-year-old girl had accused Deepak, alias Pope, of molesting her. A case was registered and the minor girl's statement was recorded in front of a magistrate.
After taking a few photographs on the bridge, the woman along with her son and daughter climbed down from the bridge, positioned themselves on a rock beside the river, and starting taking selfies, a police officer said.
The six-year-old was rushed to the Trauma Centre from school. Dr Sandeep Tewari, who was treating the boy, said the wounds "seem to have been inflicted with a knife-like sharp object".
As many as 16 people, including a noted builder, were arrested by the Kanpur Police in connection with the seizure of over Rs 96 crore in demonetised currency notes from the builder's ancestral house in Swaroop Nagar here.
Chief Minister Arvind Kejriwal on Wednesday directed officials to start installing CCTV cameras in all classes of Delhi government schools within three months, and said it will ensure the safety of children.