Wanting to right his ancestors wrongdoings during the slave-trade era, a youngboy struggles to attain peace as he unwittingly rekindles an age long war.

“Wanting to right his ancestors wrongdoings” would be a B-story from main goal – stop the war.

Stop the war is primal, simple, three word goal. “Wanting to right his ancestors wrongdoings” is complicated, confusing, thorny and misleading from the main goal. It diverts you from the main plot.

The logline is a tool, that helps you to find out potential weaknesses of your story. If you struggle so hard to put it in a simple way, I would suggest to try an another way how to tell the story.

What helps me to set things right, is slow analysis:

Kid sets things in motion (ancient war) by some action (reading a book or releasing a spell or something) question – how was he able to do that? mention that in the logline, as its a part of inciting incident and it can give us a clue what kind of movie it will be (horror, comedy, teenage fantasy like twillight, or movie for kids, like Jumanji or Hook). However, the point of movie would be probably curse? He has done it because he is curious, reckless, rebellious – and now he must become patient, grow up in order to achieve his goal

He must to fix the problem (stop the war, primal thing) – this is his goal

If he fails, something primal must happen or we (the audience) don’t care and story is bad. Someone can die, or he dies, or he loose a family in other way, or something important or valuable will be destroyed (Earth, City, something big enough to make us care).

What perhaps makes it confusing is the explanation, how he can set a past thing in motion. He travel in time back? Maybe if he won’t set things right, his own birth can be put at stake? Or destroying of Europe. One thing that can help you to narrow the story is main antagonist, who will now win a war he would have lost, because the kid altered the past?