If your child is born after the issuance of your immigrant visa s/he will not need a visa to accompany you provided you both travel within the period of validity of your visa. You are required to carry a copy of your child’s long-form birth certificate for presentation to an immigration officer at the port of entry, together with a valid travel document for the child.
If you hold a K-1 or K-3 visa your child will require a valid visa in order to travel.
Please note: If the child’s natural parent is a U.S. citizen, your child could have claim to American citizenship and his/her eligibility for citizenship must be determined before traveling