I currently found out about this topic and it has honestly stressed me a lot. I planned a trip with my family (wife + 2 small kids), I already bought the tickets (not cheap) and now I'm afraid of having to do changes.
A bit of background, my wife and I are from Mexico but live in Germany. I currently work there. My kids are 2 years and 8 months and both have only the Austrian passport.
We are travelling at the end of November to Houston, to visit family and in mid-December we travel to Mexico. There we stay until the 1st of March, when we return to Houston and leave for Germany on the 13th. What has caused me stress is that if you count the stay in Mexico, I'm staying a total of 106 days... when I really spend in the US about 30/35. I want to avoid having any issue. Besides, the problem would only be for my small kids because I am a US citizen and my wife has a visa.
On many official websites there is no information about the 90 days including Mexico. When it does, it mentiones that short trips to Mexico are included. But when I asked in the embassy they said that the time didn't matter and I would need to file for a visa. I honestly don't have the time or money for that.
I'm planning to call the CBP to discuss more about this topic. Has someone had a similar experience and could let me know how it went? I see no reason why 2.5 months in Mexico should be counted in the 90 days, besides... I have all the legal means to immigrate legally to the US if I wanted.
Update: I am Austrian, US citizen and Mexican. We all live in Germany and my wife is Mexican and has the tourist visa to the US, B1 if I am not mistaken.