AREKPOWALAN'S PROFILE

I want to make a classical coming-of-age hero's journey, but I have problem with setting the tone and story progression. Let me elaborate.

Story:
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"Arthur Smith is a 8 years old farm boy in the Kingdom of Vinland. His father is a famous knight, and he dreams to follow his footstep. He and his younger brother Lance trains himself in a local academy to become a squire. One day on a field trip, a massive earthquake hits and sends Arthur down a large hole. There, he meets a young girl named Nadia. Arthur and Lance rescues Nadia and bring her home. They find out that the girl has an amnesia, and doesn't remember where she came from. The three quickly become friends.

Meanwhile, there's an Empire that is invading and trying to conquer Vinland. Vinland itself suffers conflicts that happen within the country. Nadia's appearance causes paranoia among adults whereas the royal families start arguing with one another for petty reasons. Ultimately, Vinland's chancellor betrays the royalties and lets the Empire attack and burn down the kingdom. Lance Smith gets kidnapped by the Empire and Arthur himself watches his father bravely fights the enemy's general to his death.

Arthur himself gets rescued by his father's best friend. He and Nadia migrate to Eagleburg, where they train to become freedom fighters. 7 years later, Arthur becomes a squire. He and his comrades embark on a journey to defeat the empire to rescue Lance and defeat the Empire. But as they progress, they slowly uncover something else more terrifying than the Empire that is directly connected to Nadia."
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As you may have guessed, I have gotten this idea from "Dragon Quest 5", where you play as a kid who grows up despite tragedies to become a hero. I also want the story to be like "Lunar/Lufia", where the story focuses on childhood friend romance and down-to-earth comradeship. Overall, I want the story to be as lighthearted and inspiring as possible. But there's an issue with the story easily becoming too complicated, preachy, and most worryingly, too dark for "EVERYONE 10+" rating.

Problems
1.
The setting is based on a Medieval Europe, potentially Tolkien-esque fantasy with anime art style. It also deals with knighthood and typical good-evil fight, although the Empire won't be completely evil. The setting is perhaps the most worrying for me, because too many war stories today won't allow for much traditional heroic adventures.

Perhaps it has to do with overall perception about military-based stories. War has been something associated with darker themes, such as deaths, rapes, losses of innocence, questionable authority, and most importantly convoluted politics that pulls the story away from the main characters. Anti-war stories are very common nowadays as they keep hammering in horrible images and moral messages about pacifism which can be very preachy and annoying.

While I won't glorify war in any way, I want the story to be more about people who overcome tragedies than about reasons/consequences of war. As such, I want to make sure the story doesn't come off too angsty, paranoid, or depressing. However, I don't know how to approach this kind of theme in this particular setting.

2.
The main characterization of Arthur is based around "chivalry", which has become a dying art in the middle of the war. The kid is supposed to be a slight-book worm and dorky person anybody should be able to relate. A passionate person who likes his job and strives to become the best at what he does as he continues to grow more mature and stronger.

However, since the character is extremely idealistic - almost Superman level of heroic - I find it difficult to balance out between his optimism and his angst. The main problem about his arcs is that he grows to refuse to hurt or kill enemy's soldiers and his constant doubts to the power hungry leaders. Antagonists will constantly look down on him and he won't be able to keep being happy all the time. Most importantly, he wants to end the war in peace, but it's too easy to write Arthur's character to be angsty or to be pacifist nutcases. Any tip to help building his character and character development is welcomed.

Any help will be highly appreciated. Any question can be asked. Thank you in advance.