Protect the victim
Protecting the victim should not involve intentionally punishing the aggressor, either physically or by even frowning at the aggressor.
intervene
Be a bystander who makes a difference
If you are a bystander when a parent is coercing a child, you have to let the child know that you disagree with the coercion, otherwise you become part of an adult conspiracy against the child. Maybe you can make a difference for them too.