While separate petitions must be filed for children of U.S. citizens, it is more involved for children of lawful permanent residents.
 
When filing an I-130 petition for spouse, children can be named in that petition or a separate I-130 can be filed for each child.

It is advisable to file a separate Form I-130 for children who are likely to turn 21 in the next 5 years if you might apply for U.S. citizenship.

Children who turn 21 before their priority date becomes current will automatically drop into F2B category. However, under the Child Status Protection Act (CSPA), the child can subtract from his/her age the amount of time it took USCIS to approve the initial visa petition. You have to wait until 2 events have occurred; your family's visa petition has been approved, and the child's Priority Date has become current. At that time, you add up the number of days that the visa petition was pending with USCIS, and subtract it from the child's actual age. If that number is less than 21, the child may continue with his/her green card application under F2A. The child has 1 year after becoming eligible to submit the green card application.

If the child drops into F2B category, and CSPA does not help, it is known as the "age out" problem and he/she will face a wait of several more years. However, it is a different case when the child turns 21 after the petitioner has become a U.S. citizen.

Children who get married will drop out of the process altogether. When the parent becomes a U.S. citizen, he/she can file a new petition.