The wording in this logline is confusing.

“…the outcome of the battle between them leads their fathers to fight in the ring.”
Who’s father’s the boxing champion’s or the boxing champion’s children?

Secondly using the description ‘heir’ in a logline comes across as an attempt to increase the stakes via the use of semantics alone and therefore the stakes are very low. Heir suggests that the characters are meant to inherit a large estate or control over a land. As the boxers are still alive and well, their kids won’t get anything for a long time and at most they will get their fathers money after they die. Point is using heir is redundant best to chaneg it to their children, kids, sons or daughters.

It isn’t clear who the main character is, it could be the boxing champ that insults or the boxing champ that is being insulted. The logline doesn’t describe a main character and specific goal, without a plot it’s difficult to tell what the story.