The princess doesn't even need to be a child. Hell, make her an adult with real character and diplomatic experience. In fact, this is her 4th peace mission in 2 years, and she has caused tensions to come way down between earth and Vers. In DOUBLE fact, she is savvy and canny and despite numerous ploys by the Orbital Knights to make her look bad, or to fuck up negotiations, Princess Asseylum has almost succeeded in getting Vers to open official migration, trade, etc with earth. Aldnoah-made items (no drives, obviously, to keep the edge) in return for all kids of natural resources, water, terraforming materials, etc in the upcoming trade agreement.
The Orbital Knights (Saazbaum in particular, who owns a lot of the Martian military industrial complex) realize that their last chance for war with earth is slipping away, and nothing short of a major diplomatic incident will bring about war; even if the negotations stalled or there was a gaff, things have gotten too chummy. They throw together a last-minute plot (if they had been planning to do this for 15 years, surely it would have succeeded) to kill the princess and make it look like terrans. They seem to succeed, but fail in actuality.
The Princess and her bodyguard, Edelrittuo (who is of course this massive musclebound woman who wears a wife beater and jeans and carries all kinds of guns and shit) manage to escape, since the princess was actually in one of the black cars, not the white one. Edelrittuo is so paranoid she makes Alastor Moody look sloppy, you see. Princess is stunned but Edelrittuo activates her camo and drags her to safety with 3 or 4 armed bodyguards.
They meet up quickly with UE forces, in this case the children, and eventually with Marito's platoon. Marito in this case is Platoon Leader, since the depths of his PTSD are unknown (he hides his drinking well). Edelrittuo looks him in the eye and decides this man wasn't in on it, and reveals the secret. Of course, the 4 bodyguards are also ALSO wearing the hardlight camo and all ALSO look like mid-20's women. Edelrittuo doesn't specify which of them is the princess. She has decided she can't fuck around. Marito buys it, and his platoon decides to escort Princess to the docks, when Nikoleras intercepts them after killing the assassins. Marito finds his PTSD resurges and his entire platoon is wiped out before he gets a grip and manages to escape Nikoleras while protecting the truck carrying Princess, taking shelter in the school he teaches at (where he finds some cadets and live-fire training rounds...)
Also, we'll keep Slaine as a youth in this case, to make his infatuation with the Princess that much worse. He can still be part of her inspiration to journey into international politics as a diplomat, but before she met him, she was a capable businesswoman and politician, and only hearing about earth from him did she realize how much more she could be if she turned her mind towards the earth. Slaine naturally fell in love with her cause he's a little donger but yeah there you go.
Oh yeah and at some point, Edel tries to kill Marito because she is convinced he's an assassin. Her paranoia is her strenght, but also her weakness. They have an awesome bare-knuckle fistfight and they get all sweaty and then realize they totally are hot for each other, but neither of them trust each other, and it's even worse after the fight, which is only broken up because of a kataphract attack. Edel keeps on asking for permission to sortie in a Aerion KG-7 Kat, but due to security reasons, Marito turns her down every time (despite lack of pilots) until things get seriously bad (maybe against Ashura).
The princess, of course, is married (which makes Slain's infatuation even WORSE which is awesome) and isn't interested in any of that BS. However, she finds that her world where just being smart and clever and knowing what people want, isn't the same as the world of combat. She works as part tactician, but realizes that the rules she's grown up with aren't the same as the rules on the battlefield. Yes, she's brilliant, but there are limits. She's not trained in the ways of war. Of course, she knows many weaknesses of enemy mechs, but the Earth Kats are just so shitty it's really tough.
Just saying, PTSD-San has the potential for so much more character development than Inko, Yuki, Calm, or even Inaho (though those last 70 seconds, man, they got me right in the heart), and by making the Princess and Edel awesome instead of shit tier levels of character development.