Boundaries are important, yes, but there are naturally less boundaries between kids and their parents while those kids are still children. In order for the relationship to stay healthy as the kids grow up parents have to give what they expect to receive. Relationships go both ways.

Exactly where those boundaries lay is relationship specific, but having trust and sharing that goes both ways is always a requirement of any relationship. IMO parents sharing why something happened that had a massive impact on the kid (why a father figure left the kids life) is incredibly important - I know that I understood my parents and the events in my life a lot better once things started being shared both ways. Parents relationships aren’t wholly their own once the kids (if they’re minors) get involved.