I don't think Tim will mind if I copy/paste something from his site;
What Must Happen After Getting the Fiancé(e) Visa?
After getting the fiancé(e) visa, your fiancé(e) enters the U.S. through a U.S immigration port-of-entry. The U.S. immigration official gives your fiancé(e) instructions on what to do when he/she enters the United States. You must get married within 90 days of your fiancé(e)’s entry into the United States.
After marriage, your spouse must file Form I-485 Application to Register Permanent Residence or to Adjust Status with the USCIS office that serves the area where you live in the United States. You must fill out the Affidavit of Support, Form I-864, with the USCIS for your spouse's application to become a lawful permanent resident (LPR). See Permanent Resident at the Department of Homeland Security's, USCIS internet site.
Children Have Derivative Status
The child of a fiancé(e) may receive a derivative K-2 visa from his/her parent’s fiancé(e) petition. You, the American citizen petitioner, must make sure that you name the child in the I-129F petition. After the marriage of the child’s parent and the American citizen, the child will need a separate form I-485 Application to Register Permanent Residence or to Adjust Status. The child may travel with (accompany) the K-1 parent/fiancé(e) or travel later (follow-to-join) within one year from the date of issuance of the K-1 visa to his/her parent. A separate petition is not required if the children accompany or follow the alien fiancé(e) within one year from the date of issuance of the K-1 visa. If it is longer than one year from the date of visa issuance, a separate immigrant visa petition is required.
Remember that in immigration law a child must be unmarried. The stepparent/stepchild relationship must be created before the child reaches the age of 18.
Taken from
Adjusting status with a K1