Do I need to have 400k baht seasoned in my Thai bank account for 90 days before applying for my non-immigrant O visa extension based on a Thai child?

There are some offices that require the 400K baht to be seasoned the same as a marriage to a thai extension <- 2 months before you apply for the extension

BUT

There is nothing written into the immigration rules about that raising thai child extension that actually says the funds have to be seasoned, only that they're IN a thai bank account in your name only when you apply for the year extension.

Also no office has 3 month seasoning for ANY extension (except the raising foreign child enrolled in school here and that's for the second yearly extension onward)

Bottom line... it's BEST to visit your local Thai Immigration office and ask them for their requirements for the one year extension you are seeking.

Below are the common found requirements for the one year extension of stay as Father of a Thai child...