Generally speaking, a lawful permanent resident (or “green card holder”) only needs to file one Form I-130 (“Petition for Alien Relative”), when petitioning for a foreign spouse and an unmarried child under age 21. This is true if the child is biologically related to the sponsoring permanent resident or a stepchild of the sponsoring permanent resident. The child immigrates as a derivative beneficiary of your foreign spouse, so they can share one I-130 petition.
Keep in mind however that the child will need their own forms when it comes time to apply for their green card, separate from the spouse. If the child adjusts status in the United States, this means filing their own I-485 (“Application to Register Permanent Residence”) or Adjustment Status, or if applying from outside the U.S., their own DS-260 (“Immigrant Visa Online Application”).
Finally, remember that under U.S. immigration law, a “child” only refers to someone who is unmarried and under 21 years of age. If a petition is filed for a child but is not approved before the child turns 21, they “age out” of the child eligibility category for permanent residence.