If you're planning to get a Retirement extension, it's best to get the Non Imm O visa in your home country before you come to Thailand, then extend here. Same if you're married to a Thai. Getting any type of short-term visa aimed at tourists isn't recommended.
Children cannot be fined or banned for overstaying, but their passport does get stamped with an Overstay stamp. Not likely to be an issue, unless they plan to stay long-term and go to school here.