Hide in Plain Sight(1980)

When Thomas Hacklin goes to visit his ex-wife and his children, they are nowhere to be found. The authorities are of no help and in fact, seem to be hindering his search for them. Eventually, Thomas finds out that after his ex-wife's new husband was a witness against a member of organized crime, the family was relocated and given new identities through the witness protection program. Unable to learn the whereabouts of his children, Thomas desperately tries to locate them by himself.