The constitution (article 11) of Mexico provides an explicit right of every person to enter Mexico without a passport (that doesn't mean every person in general -- you can be barred -- but not because you don't have a passport). You can witness this at land border crossings -- I routinely watch them let in foreigners without them (they're not following whatever uncited nonsense you read at the consulate wrongly claiming a passport is 'required' without citing any law). It is subject to immigration enforcement, but they're legally barred from requiring a passport. It won't do dick to stop delinquent people from leaving and anywhere that actually thoroughly checks passports also is a member of international child support enforcement treaties.