Do I need to show 400,000 THB in my bank account again to extend my Non-O visa for dependents in Chiang Mai?

By "just got", you mean you got the visa issued from the thai consulate outside the country, right?

and

by "dependent" you mean supporting a thai child, right?

Yes you have to show the money in a thai bank account in your name only..

keep in mind, even though it doesn't specifically state that you need the funds in the account for 2 months before you apply SOME immigration offices do make you season the funds.

As long as you have a TM30 filed in Chiang Mai at the immigration office you can indeed use that office to get the extension..

make sure you go to the office and get the hand out listing the requirements of the extension based on raising thai child.

Also remember in Chiang Mai you can apply for an extension when you have 45 days or less left on your stamp