Children in Family Security Act of 2021 This bill modifies provisions related to U.S. activities to address foreign child welfare issues. The bill establishes the Office of Children in Family Security in the Department of State. The office's responsibilities shall include (1) determining whether a country that is a party to the Hague Adoption Convention has met its obligations under the convention, (2) determining whether such a country is eligible under U.S. law to participate in intercountry adoptions, and (3) developing procedures and criteria related to international adoptions. The bill also establishes the position of Ambassador at Large for Children in Family Security. The position's responsibilities shall include (1) advocating for laws, regulations, policies, and procedures that reflect U.S. policy on international child welfare issues; (2) serving as the primary advisor to the President in matters relevant to children in foreign countries living without family care (or at risk of living without family care); and (3) representing the United States in such matters. The bill transfers various authorities and responsibilities related to international adoption from the State Department to the office and the ambassador at large. The bill also modifies provisions authorizing assistance to orphans and vulnerable children in foreign countries to authorize such assistance for children in adversity in foreign countries, which shall include children who are (1) living without family care, (2) experiencing violence, or (3) living with disabilities.