If one baby / child had born in Thailand with dual citizenship ( father farang & mother thai ) he can leave Thailand for first time with the foreign passport if not have Thai passport ?

Because the foreign passport will not show any enter into Thailand, so he can live the border only with the UE passport ?

As the passport will have been issued in Thailand, yes.

But technically the child will then need the appropriate visa to re enter Thailand with the non Thai passport.

Simpler to get the child a Thai passport before leaving and use that to leave and re enter Thailand.

Thai visa question, so moving to there.

Danielsiam, the Thai passport is easy to get and it takes only a few days. Then have your child always leave and enter Thailand with the Thai passport, enter and leave the other country with the "EU" passport. This way, you have no visa hassles.

The key to exit on foreign passport issued while in Thailand would be the birth certificate. Hundreds of children leave this way (many without Thai citizenship) so it is not unusual. But as said if able to obtain Thai passport that should be used (and other only to show visa is not required to airline perhaps).

Could the baby get a Thai passport in Thailand then in my case (Im Australian) get an australian passport once they are here?

That has nothing to do with the question but suspect it would make things harder as was born here in Thailand so for most countries that would mean birth certificate would have to be verified so suspect easier if done in place of birth. Know that to be a fact for Thai born overseas and believe most other nationalities get passport from Embassy at place of birth rather than waiting for move to other country (which may never happen).