that’s wrong . You get overstay as soon as you visa expire so if you visa expire on 10 January for example and you show up at the airport the 11 January at 2am tou will be fined 500thb ( already happen to me for less than 1h past the due date. The only time you wont get fines is if you apply for an extension and your last day is a sunday , immigration are closed on saturday and sunday so some of them won’t charge you the overstay but will still put the stamp on the passport.
You will also need copie of ID card if it’s from a thai wife or birth certificate if it’s from a thai child ( depend what kind of family extension you are asking for)
You just asked for the 60 days family extension. Need to go with your thai relatives to the office and tell them to bring the ta bien ban of their house with them . Filled up extension form +tm30 and you good for another 60 days