While both the child and the mother might have the same class of agency with respect to the object level decision, they have different classes of meta level agency, that is agency about their decision how they want to choose and what kind of person they want to be. While the child does not have that agency and therefore needs to be not only protected from false decisions but also raised to that meta level agency, the mother does and therefore the decision of the son to not tell her about being drunk interfere with that meta level agency, i.e. her right to take responsibility for her own life unless the son knows that she would want to be a rational person in which case his choice does not infringe on her agency.