If you are petitioning for your spouse or fiance to come over and live in the U.S., and there are kids involved, you have a couple of options available. If you are a U.S. Citizen, you need to file separate immigrant visa petitions for each of your children. Children born abroad after you became a U.S. Citizen may qualify for U.S. Citizenship. They should apply for U.S. Passports. If the consular officer decides your child is not a U.S. Citizen, the child must apply for an immigrant visa if he/she wants to live in the U.S.
To learn more about immigration and children, you should contact an immigration attorney.