if economics are your consideration rent a car - easy two day drive to the border (can be done in one if pushing it). 5-6 days car rental is going to be about 6k give or take vs ~12k for you flights. With kids route 12 through Petchabun is elephant country and theres some nicd places to stop off.
Álvaro R. Royo you should just be able to show them a ticket. As you would understand, the reason they ask is that should a person be denied entry into Thailand the airline is held liable for flying the person back out.
If what you say is true (and it's the first I've heard of it- but I'm not suggesting that it has never happened), I'd change airlines. Of course the problem is far simpler solved - fly into Thailand with a visa. The requirement is only asked if you don't have a visa. Simple :)
Ivan McAvinchey In Australia, Privacy Act 1988 and subsequent amendments. In Europe, GDPR rules. US don't know but some states have or are introducing GDRP-style rules. Those rules are all about denying third parties access to your information without your consent. Thailand doesn't have a general privacy law (we're talking about airlines checking outside of Thailand anyway) but the constitution does recognize the protection of privacy rights. There's also the Personal Data Protection Act but last I read about that is May, not sure if that passed.
Again - it's all about people not access to your personal information.