All citizens of the United States must present a valid passport when entering Mexican territory by any means of transportation. There are no exceptions for minors. Stays less than 72 hours within the border area do not require an Official Entry Immigration Form (FMM).
Currently, a negative COVID-19 test (PCR and/or serology) is not required for entry. However, at this time, a COVID-19 viral test (regardless of vaccination status or citizenship) no more than one day before travel by air into the United States. You must show your negative result to the airline before you board your flight. If you recently recovered from COVID-19, you may instead travel with documentation of recovery from COVID-19 (i.e., your positive COVID-19 viral test result on a sample taken no more than 90 days before the flight’s departure from a foreign country and a letter from a licensed healthcare provider or a public health official stating that you were cleared to travel).