Hi,
Okay, so I need a bit more information about this:
-FATHER1
—-CHILD_A
—-CHILD_B
-FATHER2
—-CHILD_A
– Are these the posts in this hierarchy?
– ..or the taxonomy terms?
Because the problem with both approaches is, that you cannot filter terms by terms, nor posts by posts. The filters only work for filtering CPTs by Taxonomy terms or custom fields.
I think, the best solution would be, to make CPT-s, and tag them with the hierarchical terms. That way, you can use the taxonomy filters to filrter the CPT by the terms.