Server script - after insert method - listen child doctype

I have tried setup server script that should listen “after insert” action.
It seems script is not call, if reference document is child doctype.
Is is standard behavior or do I something wrong?

How alternatively “listen” insert action to childtable on server script side, please?
Listen parent doctype and look for changes in target child doctype?

Thanks for advice in advance.
Kind Regards
Jiri Sir