Relevant extract from 8CFR 221.1:
(b) Waivers.
(1) A waiver of the visa required in paragraph (a) of this section shall be granted without fee or application by the district director, upon presentation of the child's birth certificate, to a child born subsequent to the issuance of an immigrant visa to his or her accompanying parent who applies for admission during the validity of such a visa; or a child born during the temporary visit abroad of a mother who is a lawful permanent resident alien, or a national, of the United States, provided that the child's application for admission to the United States is made within 2 years of birth, the child is accompanied by the parent who is applying for readmission as a permanent resident upon the first return of the parent to the United States after the birth of the child, and the accompanying parent is found to be admissible to the United States.
(2) For an alien described in paragraph (b)(1) of this section, recordation of the child's entry shall be on Form I–181, Memorandum of Creation of Record of Admission for Lawful Permanent Residence. The carrier of such alien shall not be liable for a fine pursuant to section 273 of the Act.