What happens when the child needs to affect the parent's values?

Then the child uses the API the parent provides, typically in the form of accessors.

Or do you mean "the child cannot access the attributes of the parent directly?" Well, of course it cannot. That's what the OP wanted, and what Inside-Out objects can provide. (Note that Inside-Out objects can allow for direct access by using 'our' instead of 'my' for the attribute hashes).