A hierarchy is an artificial construct that depends on the principle of authority.
Authority is the socially-enforced rule that the ruler in a hierarchical relationship gives commands and the subordinate obeys under threat of (legitimized) violence.
Authority is a violent act. It has nothing to do with the act of rendering aid to a child; feeding them or preventing them from falling into a pool and drowning.
A parent-child relationship needn't be a hierarchy unless you construct it as such.
If I offered my boss a meal, or saved them from drowning, I wouldn't be exercising authority over them. That action doesn't create a hierarchy.
But just by being my boss, they are constantly exercising authority over me and I'm constantly their subordinate. I am ruled by them. I am constrained by the boss-worker hierarchy.
Authority is a deliberate social construct that divides people into either rulers or obeyers; using violence and the notion of "morality" to maintain this coercive system. Talking back to your boss, refusing their authority: That's a big 'moral' no no. Society uses that to uphold the oppressive dynamic and to keep you controlled and obedient.
"Voluntary" hierarchies? They're merely an excuse for perpetrating structural violence that is legitimized by appealing to authority.
That is the opposite of anarchy; it's archy.
Parenting is only hierarchical when parents choose to force their authority on their child.
An anarchist parent uses child-rearing methods that treat the child as an autonomous individual and not as a subordinate to their authoritarian demands.
Anarchist parents see themselves as caretakers, not authorities, and legitimizing parental authority with the excuse of "justifiable hierarchy" is a scapegoat. It's not justified. Using violent coercion to control children is not anarchy.
"Justifiable hierarchy" is a fundamental misunderstanding of anarchy that needs to be thrown out before it further dilutes our (really very easily defined) objectives.
We tend to overthink things and that leads to mountains of round-about revisionist theory that only detracts from anarchy and leaves people confused about what even our most basic objectives are.
Every oppressive political ideology considers the hierarchies they enable to be justifiable. Anarchists know better.
Anarchy is, was and always will be the outright rejection of hierarchy.
When you compromise and make excuses to construct hierarchies; what you're practicing is no longer anarchy.