It is one of the most usual arguments against sexual relations between adults and children that both participants have very different power. This assumed power difference between the adult and the minor does not allow a fair and mutual relationship. This argument is not based on any results about the real power situation in consensual relations.
But the main failure of this argument is the confusion of power difference and a misuse of this difference. Even in a relation with very big power difference, the more powerful partner may not use this power. Thus, a power differential does not show that the relation is unequal.
In any case, the notions that relationships must always be between peers, and that the dependent relationships are inherently unhealthy, need to be closely scrutinized.
It is a glib notion of power, however, that assumes that equality is necessary for fairness and genuine reciprocity. No two individuals are really equal in the first place, and the appearence of equality often masks manipulation and injustice. There is no evidence in logic or experience to assume that unequal relationships must be unfair and may not be genuinely mutual.