Police have said the two were last seen on security video outside their east Tulsa apartment holding hands and walking toward a creek whose waters eventually flow into the Arkansas River.
The mother of the children, Donisha Willis, 24, is jailed on charges of child neglect and assaulting a police officer. Jail records do not list an attorney for her.
The girl’s body was found in the Arkansas River, about 40 miles (64 kilometers) from the apartment, but about 100 miles (161 kilometers) away by waterway, said Terry Sivadon, a member of the task force that recovered the boy’s body, about 15 miles (24 kilometers) away by water.
Tulsa police have said the area received heavy rainfall during the weekend and the creeks and rivers were flowing rapidly.