That was very helpful! Thank you so much!
I disagree with Mr. Behar's analysis. The requirement that the marriage take place prior to the child's 18th birthday is applicable in cases where the couple is already married at the time of filing and the U.S. citizen is filing for his wife and step-child, not his fiance and her child. If the mother is coming on a K-1 fiance visa and the marriage is to take place in the U.S., the child can enter with the mother on a K-2 visa, as her derivative, as long as the child is unmarried and under the age of 21 at the time of admission.