I'm assuming you intend to apply for the Non O based on Thai child rather than retirement, as your posts are rather confusing as for which reason you intend to apply.
I also have no idea where you got the figure of 20,000 BHT as a requirement
For the Non O based on retirement, the financial requirement is 800,000 BHT.
For the Non O based on Thai child, the financial requirement is 400,000 BHT.
In either case, you must have at least 15 days permission of stay remaining for processing of the application.
You can apply for a 30-day extension to give you more time.
You posted further as I was writing;
No.
The financial requirement for the Non O based on Thai child is 400,000 BHT. I have no idea where you obtained the figure of 20,000 BHT.
The requirements are listed in item 7 here: https://bangkok.immigration.go.th/en/issuing-and-changing-type-of-visa/
Where is the Mother?
Was the child born in wedlock?
You being named as the Father on a child's birth certificate alone is not sufficient proof if the child was born out of wedlock.
Note, you must have parental responsibility and the child must reside with you as a condition.
If you can advise on your marital status, we may better advise.