If your child is losing fights consistently to the same child, you are not dealing with fighting. You are dealing with victimization. In that case, traditional conflict resolution fails. You must escalate to school administration and teach defensive assertiveness.
Characters: Protagonist could be a young leader, maybe someone underestimated by others. Supporting characters with different personalities and strengths. Maybe a tech-savvy kid, a strong but gentle giant, a quick-witted one, etc. kidsfighting.com