So the real answer is NO, you don't need an onward ticket. The immigration entering Mexico is quite relaxed.
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While the Mexican authorities will almost certainly not ask for any proof of onward travel, the airline might, and to make matters worse enforcement is not always consistent. You should be prepared to show something. Not necessarily proof that you will leave Mexico but that you are going home eventually is enough.
A few countries definitely require documented proof of onward travel. They include New Zealand, United Kingdom, United States, Brazil, Indonesia, Costa Rica, Peru, and the Philippines.
For example, they might ask you about your return ticket or proof of onward travel. They don't want to risk having to fly you back if there's an issue. You can travel internationally on one-way tickets many times and not get asked anything, then one day you can be denied boarding.
All U.S. citizens need U.S. passport books if re-entering by air. Land and sea border crossings accept additional travel documents, such as U.S. Passport cards and Trusted Traveler cards. Child travelers have additional options - see the Traveling with Children section.
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).
Airlines may impose certain sanctions including invoicing an additional amount up to the standard fare of the one-way alternative, deletion of mileage programs, etc. But, the probability of this is minimal, because such a breach can only be identified after the flights have departed.
To this day, there are no entry restrictions in Mexico and no obligation to carry a COVID test or to do quarantine. Check the current status of the pandemic in Mexico and follow the sanitary rules to prevent contagion (information only in Spanish).