How does legal guardianship work in Thailand (B1 B2 USA visa)

Hi folks. First off, it seems I rarely visit ThaiVisa unless I need something but I promise to make a few contributions to the dialog during my current stint here. Just saying I appreciate and will try to give some back...

Ok here's the story:

I am married to a Thai national and she has a current B1/B2 with 5 trips to the USA in good standing.

We have a 5 year old (her daughters daughter) that we are raising.

My wife and I want to make a another trip and take grandchild with us.

My wife and I were actually persuing adoption of said child but it was getting very complicated with me being a foreigner. We've semi given up on the process but it may be in fact the only way to get this to happen. If this is the case then I'll be back on a new topic ????

The US visa application website is pretty clear: 

• One of the child’s parent(s) or legal guardian(s) must have a valid B1/B2 (Business/Tourism) visa at the time of the child’s application. 

So what IS legal guardianship in Thailand? Is it a letter from parents giving permission? Adoption, or some other process I'm not aware of?