Support of Thai child is the same process as the marriage option, plus the childs birth certificate, blue book and ID if they have. The financials should be 400kthb in the bank the day of the extension submission, no seasoning, but its worth checking with your immigration, to be sure.
KYC takes a few inpoints. Its intended to verify you say who you are, there are some standard requirements like non tourist visa, correct name matching of your app phone company and bank account and in most cases face to face verification.
The yellow book in recent times has generally getting more accepted, but there will will be difference in offices and types of business. So for example two banks from the same company one might accepted the other the certificate of residence.