If your fiancé has children, you can apply for their children additionally, which is called a K-2 visa. Again, they have to be under the age of 21 and they are not allowed to be married. If they are, they will be on the same petition as your spouse. They will be able to enter the United States on their K-2 visa, and if you marry your fiancé, you can apply for their permanent resident status as well. When you are filling out your I-129F fiancé petition, you should include the names of the children of your fiancé along with the other required information on that form.
John Sesini is an experienced immigration attorney with offices in Milwaukee and Green Bay Wisconsin. Contact an experienced attorney to set up an initial consultation and get any questions answered regarding your specific case.