Tashawna Caldwell, 26, was arrested at the scene Sunday morning. Deputies responded to a disturbance call, where they found the child, Caiden Parker, with multiple stab wounds. The boy, who would have been two on Wednesday, was pronounced dead at the scene, the San Antonio Express-News reported Monday.
A second woman and an older child barricaded themselves for protection in another room in the house, said Sheriff David Soward.
Authorities said Caldwell made two admissions about the stabbing, to arriving officers and later in an official statement.
Soward did not comment on a motive for the incident.
Caldwell was arrested and charged with capital murder of a child younger than 6, and was jailed on $1 million bond, the newspaper said.