@Serena **************
nope it's not an extension you'd be applying for a 90 day non-Immigrant Type O visa based on raising a foreign child, you'd show that the funds were transferred into the country from abroad, you'd apply for the visa, you'd wait the 2 weeks under consideration period, then go back and get the Non-O visa and a new 90 day stamp inked into your passport
Once you had 30 days or less left on that new stamp you'd go back and apply for a year extension (by showing that the 500K baht was in the bank for 30 days before you apply for the extension)