Can a grandparent be a guardian for a child studying in Thailand on a guardian visa?

If you are over 50 and depend on what visa your child (parent of your grandchild), Non-O trailing parent to your child's legal visa/extension Could be one of your choices..

There is NO such thing as a "guardian visa" in Thailand. There is:

a. Non-O visa based on raising half-Thai children.

b. Non-O visa based on being a parent of a fully foreign child who is studying in Thailand.

There is NO way a foreign grandparent can get a Non-O visa or the extension of stay based on raising half-Thai children.

A foreign grandparent can get a Non-O visa or the extension of stay based on being a guardian to a fully foreign child who is studying in Thailand IF they have proof from their country's court that they have full custody of the child. That would require the parents to give up parental custody rights.