Concerning an alien I-130 petition for unmarried child over 21........?

My father in law has filed a petition for his son who is over 21 and not married. Here is the question: Although he is not married, he does have 3 children, ages 15, 13, and 8 yrs old. Will he be able to bring them to the United States as his dependents? If not, what are the steps to get them to the U.S., and about how long will it take? (By the way, the petitioner is a U.S. naturalized citizen.)