They were very young at the time, and had been kidnapped at the age of six, growing up in a military setting. And wouldve been killed had they refused.
ONI replaced the kidnapped children with imperfect clones, which was doubly fucked up. The clones would die fairly quickly. So as far as the parents are concerned, their children werent kidnapped. They died of health complications. But their actual children were taken by the government to become war machines.
And these war machines werent even to stop an alien threat! Several planets controlled by the UNSC were on the verge of seceding, so the UNSC created the Spartan program to squash any and all rebellion. They were made to kill farmers, essentially. Show them how pointless resistance was. Also because it was theoretically going to be easier to deploy one Spartan rather than a platoon of soldiers.
The Covenant showing up was just a coincidence, and Spartans were not made public beforehand.
Halo lore is pretty neat.
I really like the lore of Halo. I just wish the games used it more.
A god damn podcast for Halo 5 had a better plot than Halo 5.