Is there a rule when to use א when importing western loan words? We had e.g. פַּארְק and now בַּנְק (Yiddish has באַנק of course).
Not that I'm aware of. However, perhaps in cases where the loan word creates ambiguity, א is used - for instance, פרק is a chapter/episode, but פארק is a park.
Is that one account for each child or a single joint account?