respectfully, are you nuts? Your child would still be illegally in the country. Plus you (well, they) can be denied entry in the future even if the child can’t be fined. Wth!?
nothing lasts forever in Thailand. The 60 Day exempt is temporary and will at some point go back to 30 days. Running to immigration for extensions and paying money is a pain in the neck, total waste of time/nerves/money. Sure, tourist visas are possible but they also cost a processing fee. If he can get the documentation, do the DTV and then no headaches for 5 years.
may I ask what the benefit of that is? You have kids who can only stay a very limited time and you/husband can stay 180 days. How are you planning to deal with that issue?