According to Sugar Land Police, 18-month-old Elena Barras and 4-month-old Emmanuel Barras were seen around 12 p.m. on Tuesday, May 3.
Officials said they were taken by their father, who is not their primary caregiver, under suspicious circumstances.
Police said their father, Taylor Hudson-Oneal, was visiting his ex-girlfriend and their two children when he unexpectedly left in the ex-girlfriend's car, taking the children with him.
Hudson-Oneal does not live with the children and is not their primary caregiver, police said. He reportedly has limited contact with them.
Police did not make clear in a Facebook post about the disappearance whether Hudson-Oneal was taken into custody or if he was still at large.
Investigators say Hudson-Oneal is known to frequent southwest Houston and the Sunnyside neighborhood.