Migrant Child Trafficking Prevention and Accountability Act of 2021 This bill imposes sanctions on an alien parent (or guardian) of an unaccompanied alien child who arrives at any U.S. border or port of entry. Under current law, an alien may arrive in the United States (whether or not at a port of entry) and request asylum, subject to certain restrictions. If the parent is not a lawful permanent resident, that parent shall be permanently barred from adjusting immigration status and deemed inadmissible into the United States. In addition, such a parent shall be deemed to have violated certain crimes related to assisting or abetting an alien's attempt to enter into the United States in violation of immigration laws, even if the parent's actions would fall under an exception for a first offense in assisting or abetting certain family members. If the parent is a lawful permanent resident, that parent shall be ineligible for any federal public benefit.