In "My Hero Academia" (MHA), Mirko, also known as the Rabbit Hero, is portrayed as a fierce and determined character, but there is no indication that she hates kids. She is a professional hero who is dedicated to fighting for justice and protecting people, including children.
Her tough exterior and intense personality might give some the impression that she is harsh or brusque, but her actions throughout the series suggest that she cares about the safety of all civilians, regardless of age. As with many characters in MHA, there can be nuances in their personality and behavior, but a clear hatred for kids is not part of Mirko's characterization.