i would advice her to go to Canada on holiday with the kids ...NOt suggesting she takes the kids illegally, get all the permission etc....
....Also try get a good lawyer to be involved in the early phases so that she doesnt make ANY mistakes. It is so easy to do something emotionally that can mess everything up. Keep to the book!!
This is so typical. Nearly every couple in this configuration ends this way. Gringa women with Mexi men are a recipe for abuse. the women have no family present to protect them nor experience nor experience in defending themselves. Anyway, many Mexi women accept a certain level of abuse, it's still somewhat "normal" here, as is having multiple "wives" and families.
 Yes, get her out of there. Getting all the permission is the big issue. First, check what is required for her to enter Canada with her kids. Is husband's permission required?  Probably, I think it's international law now.
The big question is if the father is on the birth certificates.
So, here's the approach;
1) Get her out of Cancun. Far away from him, Oaxaca, Chiapas or further.
2) Get a lawyer at the new place to file for her to have sole legal custody. They have another name for this.
3) When he files, the reason for sole custody is going to be that the father has disappeared and cannot be found. Use a nearby false former address for the husband (coordinated with residents at the address) that the judge can send his investigator to.  There, they will say that the man left long ago and has not been seen. The investigator and judge will likely accept it with no concern.
Yes, this is deception - sometimes, especially in Mexico, it takes deception to get justice.
4) If the lawyer is good enough and the judge is causual, as they usually are, she can get sole custody and leave the country.Â
An alternative would be to live in a marginal area for a bit and try to take out late birth certificates for the kids that have only mom's name. This is still typical in Mexico, as poor people (and hippies) don't always register the birth right away. Two, three years late is not unusual. Need a midwife to cooperate, perhaps.
If none of this is acceptable or workable, the laws in Mexico are now very protective against battered women and children.  Problem is that the system is so corrupt that she might end up in jail before him. Depends on his power, connections, money, the area, the judge etc. I would only pursue it like this as a last resort. Â
Good luck,
Steve