So how do you make a weak reference? You stick a WeakArray in as a holder.  So in your parent/child objects you would write something like:

Node>>parent
       ^ parent ifNotNil: [parent first] ifNil: [parent]

Node>>parent: p
       parent := p ifNotNil: [WeakArray with: p] ifNil: [p]

thus using a WeakArray as a holder for the parent.

When a weak reference gets garbage collected, its weak references are set to nil first.