No problem to leave before his non-b expires and return on exempt entry. But he should cancel work permit with local labour office.

13 yr old cannot be charged overstay or does not have to worry about ban for overstay. So, just have the daughter enter on exempt entry and don't worry about it.

Side note for the future.... if you decide to leave while on extension of stay you need to get a re-entry permit. If you quit your job or decide to leave permanently, you have to cancel the extension of stay before you leave. Good luck.

1000 baht for re-entry permit. It does not pause your visa so the admitted until date remains the same. So... if you'll have 30 days or less, better to just leave and come back in on visa exempt if you're eligible.

You can do it in any city with an immigration office. What city will you be staying in?

Which imm office?

Paul is right on but, let me also comment that you do not have to go to immigration at all for the six months. You can just do border bounces every 60 days and get another 60 days (stamp out, go across, stamp in, turn around, come back into Thailand). But it really depends how close/convenient a border is. Or just plan a little trip to Malaysia at the 60 day mark. No need to buy a new visa... just go and come back and you get 60 more days. Your visa will have a 6 month validity from the date of issue so you can even go on the last day the visa is good for and do another border run to essentially get 8 months out of the visa (which can also be then extended by 30 days). Cheers!

Also always check the consulate website for holidays when planning your trip so you don't show up and find they're closed.

If you don't extend it with the proper paperwork from the school, it will expire Nov 25 and you must leave the country before that date. You don't have an extension or a work permit so you can leave without cancelling anything and stay until that date.

30 days