Just a quick post about our experience bringing our 12 year old niece from Vietnam to Thailand. Something we've not done before - traveled with a child, and also not our own child. We got the Viet government form, which lists the Mother & Father, has my wife's information too. All notarized, which we had translated to English, and notarized. Checking into the Tan Son Nhat airport, the Viet Air desk, or Immigration, never checked anything but her passport. When we arrived at Suvarnabhumi, immigration never asked either. Must say I was a bit surprised, but happy.
TLDR : Answer Summary
A traveler shares their experience of bringing their 12-year-old niece from Vietnam to Thailand. They had all required documentation (including a notarized letter from the parents) but found it surprising that neither the airline nor Thai immigration checked these documents, only focusing on the child's passport. Commenters discuss the importance of carrying documentation, especially when returning from Thailand, as the checks may be stricter.