The children were last seen at their maternal grandparents' house, who have court-ordered custody of the children.
The grandparents live in the 38000 block of Kyle Place in Palmdale.
Sheriff's believe the mother, Teresa Roy, took the children at about 10:00 a.m. on Thursday.
Deputies say the children were placed in their grandparents' custody because the mother had a history of neglect.
The location of the children's father was unknown on Thursday night.
Authorities say the mother had a verbal argument with the children's grandmother before the kids disappeared.
Officials are looking for a blue 2000 Toyota Sienna Minivan with gray trim. The license plate number is 4HYR792.
The boy is described as a 5-year-old black male, 3 feet 8 inches, 48 pounds, with brown hair and brown eyes. He was last seen wearing a red t-shirt and blue jean shorts.
The girl is described as a black female, 4 feet 10 inches, 70 pounds, with long brown hair and brown eyes. She was last seen wearing pink pants and a yellow top. The girl wears glasses.