Is it correct that children under 14 are required to have their own visas but if they don't, immigration officials leave them alone? No need to pay overstay fees, no blacklisting?

I hear a lot of farang families on tourist visas don't bother getting one for their kids. I guess the letter of the law is that kids need to have visas but in reality Thai officials don't care (as of yet)?

Yes, children do need their own visas and extensions of stay.

It is true that immigration will not arrest or fine children that have not turned 15 yet. But overstay is still noted in their passport and the best way is of course to keep them fully legal.

True, children can not be fined for overstay.

They will however get a stamp in their passport.

Thanks, good to have that confirmed.