What sort of ID is required for minor children to fly with parents inside Mexico when the children are a mix of US citizens and Mexican citizens, and the parents are Mexican? (Three of the children are US born and do not have dual citizenship.)
There are various regulations regarding children travelling with neither parent (eg) but there appears to be nothing specific for those travelling with at least one parent.
For example Suzanne Barbezat at the same source wrote:
Every traveler arriving in Mexico by air, regardless of age, is required to present a valid passport for entry into the country. Mexico does not require passports to be valid for longer than the anticipated length of the visit. Children who are not Mexican citizens are not required by the Mexican authorities to present any other documentation other than a passport.
It would seem odd to me that any paperwork not required to cross a border would suddenly become required once inside the country.
Mexican citizens (including dual citizens of other countries) who are under 18 years of age and traveling unaccompanied by at least one parent will need to present proof of the parents' authorization to travel.
The authorization from the parents (required by law for Mexican nationals only) must be translated into Spanish and legalized by the Mexican embassy in the country where the document was issued. Read more and see an example of a letter of authorization to travel.
Canadian Children Traveling to Mexico
The Canadian government recommends that all Canadian children who are traveling abroad unaccompanied by both their parents carry a consent letter from the parents (or in the case of traveling with one parent only, from the absent parent) showing the parents' or guardians' permission for travel. Although it is not required by law, this letter may be requested by Canadian immigration officials when exiting or re-entering Canada.
Leaving and Returning to the U.S.
The Western Hemisphere Travel Initiative (WHTI) establishes document requirements for travel into the United States from Canada, Mexico and the Caribbean. The travel documents required for children vary according to the form of travel, the age of the child and whether or not the child is traveling as part of an organized group.
Travel by Land and Sea
US and Canadian citizens aged 16 and over who are entering the United States from Mexico, Canada or the Caribbean by land or sea are required to show a passport or alternative WHTI-compliant document such as a passport card. Children up to the age of 15 may present proof of citizenship alone, such as a birth certificate, a consular report of birth abroad, a naturalization certificate, or a Canadian citizenship card.
Given the amount of detail in the above I deduce that lack of mention of ID requirements for minor children to fly with parents inside Mexico is because there are no such special requirements. It is possible that specific airlines may have their own regulations but in that case it would be a matter of checking the T&Cs or asking whichever airline is involved.