the three-year-old boy picked up gun inside a cleveland , ohio , home .
he then pulled the trigger , shooting and killing braylon robinson .
it is unclear whether victim was related to the youngster who shot him .
baby 's mother could be heard screaming on the back porch on sunday .
police are trying to determine who left weapon within the child 's reach .
force chief said of accidental shooting : ` this is a senseless loss of life '