Are you in your home country or in another country outside Thailand? You can use a utility bill or your driver's license in your home country or a visa entry stamp in the child's passport if abroad.
It's known as a retirement visa, but officially the required criteria is being 50 years or older, you don't need to be retired or need a pension. You just have to follow the funding requirements to the letter.
They aren't f#kn around they want 100% specific proof, you'll need to supply exactly what they asked for, a closeup of the receipt and another obviously Vietnam shot in a location
I also live in Chiang Rai, use Mae Sai office, my understanding is they don't care about this for trips around thailand if you are returning to your main residence.