well aware of that been living here long enough. But he is UK passport so doesn't get VOA. Also mentioned 60 days so obviously not from a country that gets 15 days wasn't hard to work that out.
the TDAC if that's what you think is the online check-in does not guarantee entry. If the Immigration officer decides your not entering then your not. Even with a proper visa they can refuse you entry although it's very rare they do.
On the TDAC You don't need to enter departure date so they have no idea how long your coming for.
I ask why do you have criticism for Thai immigration?
Look at your own country rules will they allow you to have your son with you because as you keep saying your a dad they should have compassion. Your govt don't care either same as Australia govt don't care. You must do relevant paperwork for an issue you sadly have in all countries.
difficult to open without long term visa married or not. You get OA or OX visa when in Australia they are already multiple entry. Get non O then year extension you buy multiple entry at the airport after you have it.