The mother, a 26-year-old policeman's wife, was pushing the child in a stroller in the city of Bryansk when the ground caved in underneath them.
Police say the woman was rescued, but the child was swept away by the current into a collecting tank for the sewage system and was presumed dead.
Investigators say they are working to determine the cause of Sunday's accident, but suspect faulty construction of the system may be to blame.
Underground pipes carrying hot water occasionally burst in Russia, causing the ground above them to suddenly collapse to the peril of those walking above.
