Bringing Children of Permanent Residents into the U.S.
A lawful permanent resident (a foreign-born individual with a green card) may petition to bring child to the United States. Under U.S. immigration law, a "child" is a son or daughter under the age of 21 who is unmarried. Permanent residents also may petition on behalf of unmarried sons and daughters who are over the age of 21 (as well as their children).
For a more complete overview of this subject, see "Bringing a Child to Live in the U.S." Below is supplementary information for permanent residents.
In addition to the standard documentation requirements (see related article, linked above), permanent residents must present proof of lawful permanent residence. To do so, submit a copy (both front and back) of Form I-551 ("green card") or a copy of your foreign passport with a stamp proving permanent residence in the U.S.
Filing on Behalf of Relative Living in the U.S.
Lawful permanent residents petitioning on behalf of a child (unmarried and under the age of 21) or unmarried son or daughter over the age of 21, if living in the U.S., must file a Petition for Alien Relative (Form I-130). Your child, son or daughter may file an Application to Register Permanent Residence of Adjust Status (Form I-485) when a visa becomes available.
Refer to the USCIS page "Bringing Children, Sons and Daughters to Live in the United States as Permanent Residents" for more detailed information.