Elite of you have cash and don’t need to work. 20 years 45 grand. If you work remotely and have 80k USD a year in income and have 1 million USD, you can buy 500k USD property and get 10 year LTR that allows work.
It’s Article 21 of the Convention on the Rights of the Child (CRC) and in the 1993 Hague Convention on the Protection of Children and Cooperation in Respect of Intercountry Adoption.
She should be carrying both passports. Enter on Thai. Leave on Thai. Enter US on US. Leave on US. If any immigration officer asks, she only has to tell them she’s a dual citizen and traveling between her countries of citizenship.