I just read last night that Mexico has a new law regarding traveling to their country with children.  In an attempt to cut down on trafficking/kidnapping events concerning children, it will now be necessary to bring additional paperwork showing your relationship to the child/children that are traveling with you on your vacation to Mexico.  Since the law is so newly enacted, there is sparse information about exactly what paperwork this would entail.  So, until Mexico becomes more clear about that aspect this is what is being advised:

• bring your child/children’s birth certificate showing parental association (along with their passport).  While the original is best, for right now a certified copy of one would do.
• if last names are different (stepparent association for example) bring paperwork that establishes your relationship to the child (I imagine it would be the marriage certificate if it’s a stepparent situation).
• grandparents taking their grandchild on vacation without the parents would need a notarized document giving parental permission to take the child into and out of the country.
• bring custody agreement papers if that is appropriate.
• call the resort at which you would be staying and ask them directly what their requirements would be.

I will continue to monitor this situation and let you know what Mexico decides is a more definitive answer to this question.