@Jyrki ********
I wouldn't recommend burning through your 60 day extension of stay based on visiting Thai Family UNLESS you're ready to get a one year extension of stay based on marriage/ raising half-Thai children.
To get a 60 day extension of stay based on visiting Thai family, you will need the following:
1. Your passport and a copy of:
- Your data ("picture") page
- Your Non-O visa
- Your entry stamp
- Your departure card
2. Your wife OR your half-Thai child PHYSICALLY next to you, and
- Her ID card and a copy of her ID card [for Thai wife]
- Her housebook, and a copy of the first page [for Thai wife]
- The child's birth certificate [for half-Thai child]
- The child's house book [for half-Thai child]
3. Marriage certificate (either the rose border certificate, OR a translated copy from your home country) [For Thai wife]
4. The application form
5. 2x passport pictures
6. 1,900 THB for the processing fee.
Different immigration offices have different requirements, and this may include:
- A copy of the TM30 filed where you live
- Rental contract (if renting property or a condo)