Meet the aggressor where she is, without resistance, as opposed to disapproving from above; see it from her PoV; what was this about?; what led up to this? How can we proceed positively from here?
When a toddler hits a parent, should the parent communicate their honest reaction, whether it be showing hurt if they’ve been hurt, or any emotional response, such as feeling anger or sadness?
Protecting the victim should not involve intentionally punishing the aggressor, either physically or by even frowning at the aggressor.
Children have to do what they themselves think is right, with no pressure whatsoever—that’s what non-coercion amounts to—but they also have a right to be told morality as best we see it.