if the OP move to another country and not plan to come back, just leave by airport and the current extension (without reentry) will be voided. However if you plan to come back and start taking up Employment than you have to void work permit and extension before you leave.
it's a bit weird way to look at the kids situation. Legally everyone need to have visa or an extension of stay. However Thailand does not fine Children under 15 years of age for not maintaining the "stay". This has nothing to do with the status of parents, it is courtesy to obey the law of host country.