Function hierarchy_iter_depth_first_child_to_parent 

pub fn hierarchy_iter_depth_first_child_to_parent<R, F>(
    q: &EntitiesHolderQuery<'_, (EntityId, Option<&ChildComponent>, Option<&ParentComponent>), F>,
    f: impl FnMut(EntityId, &[EntityId]) -> R,
)
where
    F: QueryFilter,

f(optional parent, children)