What are the requirements for extending a one-year visa with children in Phuket?

This is from the Phuket Immigration Volunteer website and is pretty complete as far as what you need to get a year extension down there at that office

You are using ONE child to qualify for you holding a visa/extension based on the parent of a foreign child on an ED visa/extension here in thailand and that requires 500K baht in a thai bank account in your name only for 30 days before you apply for the extension for the very first year extension you get and then for every subsequent yearly extension after that you need to have the 500K baht banked for 3 months before you apply for the extension.

You only need ONE child to qualify for that extension you're getting, that you have more than one is neither here nor there 😉