I agree. My son, US citizen travels with me regularly from Thailand to the US. I've always utilized an (English Language) notarized consent to travel document. However, our last departure in April, immigration told me that I would be required to provide a document in Thai and procured from the Ampur. Why, not sure.
As many have stated, have both passports for your travels. Also, ensure the mother knows the travel plans, day and time of departure in the event immigration decides to call her. They always call my son's mother to verify.