The invading aliens were sort of slavers who invade planets, take only the children, and kill everyone else. They then implant devices on the children (I think they were bombs implanted to the head/neck) to control them. Then they train them into merciless soldiers to fight their wars for them.
I recall very few details of the story other than the overarching plot. I do remember one detail though, when the aliens were herding the children into the organic alien ships. There was a brother and sister, who I think were the main protagonists, they were being sorted based on gender, age, and so on and separated into "rooms" via slimy passageways. I remember they were very young.
They were then trained to be soldiers for the aliens in brutal training camps through their teens. It had a YA feel to it. I think it might also involve some sort of superpowers that the aliens were looking for, but I'm not sure. But they become battle-hardened soldiers, even though they were quite literally in their teens. Anyway, the children eventually broke free, stage an uprising, and wreak vengeance on the aliens who killed their parents and their world.
I remember coming across this story on link of recommendations on Imgur about scifi stories featuring "vengeance" plots about alien invasions and humans reigning supreme. I don't know if it was the HFY ("Humanity F Yeah!") genre, but it was very similar. I also remember, I first learned about John Scalzi's Old Man's War series through the same link.
I'm not even sure if it was a book or amateur free fiction. I remember it being quite long. Separated into multiple chapters.
