I-130 is what you should start the process with. The I-485 is the application to adjust status to greencard holder, but that is only if a visa is available. Since you are only a greencard holder there will be no visas immediately available. They will have to wait several years. However if you naturalize you will be able to petition for your child under 21 immediately. Your children also need to stay in lawful status, especially the older one.
You should work with an immigration attorney. Immigration law is very complicated. You may call me anytime for a free legal consultation at 206 724-1940.
I agree. It is currently taking 4-5 years for children of lawful permanent residents to obtain an immigrant visa (green card) according to the Visa Bulletin. If you are eligible to become a citizen, you should do so before applying for your children so that they do not have to wait so long. If you have been a green card holder for 5 years or 3 years if you are married to a U.S. Citizen, then you may be eligible for citizenship.
Even if you are filling out the forms yourself, you should at least consult with an immigration attorney before filing anything. Please feel free to contact me to discuss this matter further.
Seeta L. Nangia, Esq.
Law Offices of Seeta Nangia
Exclusively Practicing Immigration Law
Phone: (415) 273-9123
Your question cannot be answered properly without more information. How did you get your green card? How are your children in the U.S. legally? For example, if you obtained your green card through marriage to a U.S. citizen, the child under 21 may be immediately eligible for a green card, as long as the marriage took place before he or she was 18 years old. Please call my office if you would like to discuss further.