There are a number of options. These will depend on the ages and marital status of his children. Those under 18 at the time of his marriage to his US citizen wife should be petitioned for by his wife. They will be classified as immediate relatives of US citizens and their visas will be approved relatively quickly because visas for immediate relatives of US citizens are not subject to quota and visas are always available for immediate relatives. Assuming they are unmarried your father should petition for the other children directly. under either F1A or F2B visa categories. But in the end, I suggest you consult in detail with an experienced immigration attorney.
Answered on May 20th, 2014 at 6:29 PM