The K-2 non-immigrant visa allows a child of your Fiance to enter into the U.S. and await the availability of an immigrant visa. K-1 children may stay in the U.S. on K-2 visa for a maximum period of 90 days from the date of entry. If you and your Fiance do not marry within 90 days, K-1 children must depart the U.S. together with your Fiance.
The applicant must be the unmarried child under 21 of a person who qualifies for the K-1.