No, all the code is executed only after $(document).ready() and even few secs later. It would be too easy :).

I also noticed than my custom module is in 'activit' folder and the same name has the folder where my theme is. Maybe that is the problem.

I cleared the cahce multiple times... I just checked my previous projects and realised that in every project I've made (D7) I used arguments and file template. It is the way I am used to. It allways worked. I also tried what you said - to use variables but it still just does not work when the theme is defined as a function theme_activit_package_teaser() :(

That's weird because now I have the theme html in .tpl file and it works even if i still use arguments. Check update of my question.

I have tried to to use .click() on select and then .click() on the option DOM element but it does not work on select element: stackoverflow.com/questions/430237/…

Yes, I've tried multiple ways including this one, but it all works only when typed in browser console (maybe in .js file too), but nothing works if the code is executed by my chrome extension. It is weird... does Drupal somehow verify what exactly triggered the event? Is it even possible?

Nope. The command is in a function that is executed by setTimeout(), but that should not be a problem. I'm lost now.

Probably you're right. I just found that if I type directly to console command $('#children-div select[name="number_children"]').val('3').change(); it will properly trigger a change event. But if the same command is executed by my chrome extesnion, for some reason it changes the value but doesn't trigger an ajax.

No, there is nothing special in setup/code. Maybe there is more complicated drupal JS mechanism that is triggering ajax than a single change event.

Tryed, but no effect.