The K-2 Visa Adjustment of Status (AOS) Application Process
If the K-2 adjustee is under 14 years of age, they will also receive a biometrics letter but are only required to attend for photos.
When sending payment, you can either write one check for each application or one check for them both. The benefit of writing just the one check is that if for any reason the K-2 AOS paperwork is not accepted and therefore returned, the full adjustment fee becomes payable. However, if the paperwork is not accepted and the check is for both applications, both applications will be returned and the correction to the K-2 application can be made and resubmitted for the same fee.
Each K-2 applicant will need their own form and supporting documentation.
I-864 - Affidavit of Support: K-2's don't require their own separate I-864 with supporting documents (tax returns, etc). Make sure they are listed in section 9 of the K-1's I-864, and provide a photocopy of the K-1's I-864 with the K2's paperwork. You don't need to include copies of the supporting documents with the K-2's paperwork. You won't get an RFE if you do happen to provide a separate I-864 with supporting docs for the K-2. It's just not required.
I-765 - Employment Authorization Document: This is one of the two documents that the SSA will request if you want to apply for a SSN for your K-2 adjustee. Applicants under age 14 may also apply for one, if you wish to ensure that you can get a SSN for your child before their GC is approved.
I-131 - Advanced Parole Works in exactly the same way as the principle applicant's AP.
Older K-2 Visa adjustees
Child Status Protection Act: Immigration law generally requires a derivative child visa holder to adjust status before they are 21 years old. The Child Status Protection Act protects them from "aging out" as long as their AOS petition is accepted by USCIS before their 21st birthday. However, the CSPA only applies to K2's if the K1 marries the US citizen petitioner before the K2 turns 18. If the marriage occurs after the K2 is 18 then their AOS must be approved before the K2 turns 21.
This can be a big problem because a K2 is eligible to receive a visa right up until they are 21 years old, but if it's too close to their 21st birthday when they come to the US then they might not have time to finish AOS before aging out.
Follow To Join
A qualifying child has a window of one year after the K1 visa is issued to obtain a K2 visa at a US consulate. If they don't obtain a K2 visa within that time then they are no longer eligible for a K2 visa. However, they may be eligible for an immediate relative or family based visa based on their relationship to the US citizen or the K1 parent.
If the US citizen and K1 marry before the child turns 18 then the US citizen can petition for the child. If the child is not married and under 21 years old then the US citizen can petition for an IR2 visa. If the child is not married and 21 or over then the US citizen can petition for an FB1 visa. If the child is married then the US citizen can petition for an FB3 visa.
If the US citizen and K1 marry after the child turns 18 then the K1 can petition for the child after the K1 gets their green card, as long as the child is not married. If the child is under 21 then the K1/LPR can petition for an FB2A visa. If the child is 21 or over then the K1/LPR can petition for an FB2B visa.
K-2's Status and Rights Post-Adjustment
A K-2's status is entirely dependent on the K-1's status. If the K-1 doesn't marry the US citizen petitioner then neither the K-1 nor the K-2 can adjust status. If the K-1's AOS is denied then the K-2's AOS is automatically denied. K-2's are conditional residents for the first two years, just like K-1's. K-2's need to apply for removal of conditions just like K-1's. If a K-1's removal of conditions is denied then the K-2's is denied, as well.
There are some exceptions to the above. A K-2 can't apply for US citizenship after three years, while a K-1 who remains married to the US citizen can. A K-2 has to wait five years, just like most other LPR's.
Once a K-2 successfully removes conditions then their status is no longer dependent on the K-1 parent's status.
For more information please see the following forum thread.