If the child is not named on the I-129F petition, will that be a problem?

The K-4 visa will not be denied because the child's name is not listed on the I-129F petition. As long as it can be established that he/she is the minor, unmarried child of the applicant issued a K-3 visa.

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 long 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.