Quick author notes:
This story takes place pre-game, so this is obviously pretty spoiler-free for those unfamiliar with the game and it's story, so don't worry.
Also, for reference, Reyn, Fiora, and Shulk are all six, while Dunban is eighteen.
Now, to the story
Xenochildren: The Boy Princess
A small auburn haired boy is running through the field just outside Colony 9, a small wood shield strapped to his arm, a wood sword in hand. He's running around scaring the bunnits as he waits for the "hero" of his little game to show up. He hops onto a rock and stands proudly, as if nothing could go wrong.
"Reyn!" A cute little girl voice shouts to him, "Are you ready?"
Reyn turns around and smiles brightly, the gaps in his teeth clearly show where he's lost some. He hops down off his rock and proudly attempts to march to his friend.
"I'm all ready to start Fiora!" Reyn replies as he salutes, a lisp obvious in his voice, "Wait, aren't you the Princess? Where's Shulk?"
The blonde shakes her head in defiance as she drags out her two wood daggers, holding them upside down, "Shulk wanted to read so I'm the hero today! He's going to be my princess!"
"I thought princesses had to be girls."
Fiora stops playing around with her daggers like they were batons, "Well… um… he can be a boy princess!"
The boy smiles brightly again in agreement as he raises his toy sword high above his head, "Just so you know, I'm not gonna go easy just because you're a girl."
The girl giggles as she tosses a dagger in the air and catches it, "I won't go easy either."
Both strike a simple fighting pose, Reyn being loud and proclaiming his is the strongest of all Mechon. He hams it up as he rushes in and playfully hits Fiora with his shield, saying she'll never defeat him. She counters in a similar manner, saying the power of love will prevail over a Mechon any day, striking her friend's shield with her two daggers before tripping him, taking this advantage to run towards Outlook Park, where her 'princess' awaits her.
Recovering quick Reyn takes off after the girl, playfully tackling her to the ground before being thrown off and a having a dagger pointed at his face.
"Vile Mechon!" Fiora says, copying her brother, "You think I, Fiora of Colony Nine, will surrender to you!?"
The auburn-haired boy slaps the dagger from his face with his sword before standing and pointing at her, "You're a tiny Hom compared to me! I will never let you win!"
The two start to playfully fence as the blonde is pushed backwards towards the park. They continuously throw witty child banter until Fiora knocks Reyn's sword out of his hand and she pretends to stab him, proclaiming her victory. The boy pretends to fall over dead and let the girl have her fun as she runs to the top to 'rescue' Shulk.
"Fear not anymore fair princess!" Fiora proclaims as she reaches the top, holding back a giggle as Reyn peeks from the top of the stairs, "Your hero has defeated the Mechon!"
The blond boy looks up from his book and smiles, he wasn't ever fond of playing this game, and didn't know how to react to being 'rescued'. The child crawls off the bench and walks over to Fiora.
Shulk smiles and shows off some of his own tooth gaps, "Thank you, my hero."
Fiora leans in, "Now, a kiss for your hero? Please princess?"
The boy blushed as he hears his friend start laughing in the back. Just as he was going to lean in, a much more mature voice intervenes as the auburn-haired boy's laughter ceases.
"Here I thought princesses were girls." The raven-haired teen says as he picks up Reyn and let's him ride on his shoulders, "I thought Shulk was supposed to be the hero as well today."
"B-but Dunban! I wanted to finish reading!" Shulk whines.
"Reyn and me agreed Shulk could be a boy princess too." Fiora huffs as she puts her dagger away.
Dunban lets Reyn remove his headband from his forehead and put it on as he picks up his little sister and her friend in his arms to carry them down to the colony again before night falls, "I do want to ask though, why does Reyn always have to be the Mechon when you two can switch off being hero and princess?"
Reyn decides to talk after adjusting the headband and stuffing his sword under his shield, "I choose to be the Mechon Dunban! I want to be strong like you! So being the Mechon makes it so I don't have to wait around for the hero as princess or be easy with the other two as hero!"
The oldest chuckles as he reaches the base of the stairs, "Of course you want to be strong Reyn, maybe when you get bigger you can join the Defence Force like I did. Now, do you and Shulk want to go home or stay with me and Fiora tonight?"
Shulk ceases fiddling with a loose tooth to look at the other, his blue eyes full of hope, "If we stay are we going to have curry? I really like when we have curry."
Fiora nods happily as she asks Dunban to drop the trio off at the house to go get Giorgio's curry for the four of them before he closes for the evening. All three of the young kids cheer as he chuckles and agrees because then it means he doesn't have to worry about Fiora burning herself again while cooking, and he doesn't need to bother attempting to cook.
That... is probably the cutest thing I've written in a while. I will be honest that "Hero, Mechon, Princess" is not my original idea. I say something on tumblr talking about it and I thought it would be adorable to write out. Also, I like the idea that Reyn lost most of his teeth due to stupidity, not because he's six and that's a normal occurence, compared to Shulk missing a few, and Fiora hasn't lost hers yet.
I also see Dunban acting as their babysitter when he's not being drilled by Vangarre in the military district of Colony 9 (Judging that Reyn is a "recruit" in XC at 18, that's the minimum age to enlist in the Defence Force, Dunban will just be starting out and he hasn't started using the Monado yet.)
If there's any cute idea what could take place for the Colony 9 Trio while between the ages of 5 and 15, feel free to let me know.