The children were reported missing about 1:30 p.m Monday from their Diamond Bar residence east of Los Angeles, sheriff's officials said.
"We are looking into the disappearance of the children," sheriff's spokesman Steve Whitmore told The Associated Press.
Investigators were looking into whether the children were taken by their "non-custodial mother," said sheriff's spokeswoman Deputy Aura Sierra, referring to a parent without custody rights.
The children's parents are divorced and appeared to be in a custody battle, the San Gabriel Valley Tribune reported on it Web site.
Messages left with Miller's local offices were not immediately returned Monday night.
Miller represents the 42nd Congressional District, which includes the communities of Diamond Bar, Brea, Chino and Chino Hills.