Don't really have to worry about children until their 14. There's no overstay fine for children and won't effect them. Just make sure they get a new passport after they turn 18.
Sorry to hear your predicament Paul. Even with my extension, my stamp expired April 11th, but this round of covid extension started the day you applied, so I lost about 2 weeks. This seems to be consistent with the other reports of people I'm reading too.
The new orders from 25th of January give discretion on how each office applies it, and when the extension starts seems to be the area where the greatest discretion is being exercised.
For all extensions, Bangkok allows you to apply for an extension up to 45 days before your stay. You still should have been able to apply for an extension, but it would start from when you apply, so you'd loose a whole month. Do you have any marriage documents for China? If so, you could see if the Chinese embassy would issue you a letter that you cannot travel to China, might be worth a shot.
"Note: Although International Driving Licences are valid for up to one year after arrival in Thailand, after three months many comprehensive insurance policies are made void if still driving on a foreign licence."