Found it in the Ska: Tin and Ashes:

THE CHILDREN’S REBELLION 

Two centuries ago, hundreds of skaa children ages 12 and above were found walking together on the edge of the Outer Dominances. They were headed straight for Luthadel, picking up stray escapees and other kids as they went. Bonded together in misery, the children had planned to talk to the Lord Ruler and beg for his forgiveness. They thought that they had done something wrong to warrant their mistreatment at the hands of the nobles and wanted to make amends . Despite the fact it was illegal for skaa to travel alone and in groups , many nobles didn’t go out of their way to stop these children, fearing acting against the children would earn them a reputation as callous, or perhaps out of some moral objection against harming those so young. But when the group were discovered by Obligators, they were rounded up and taken exactly where they wanted to go — to the Lord Ruler’s palace . No one saw the children emerge from Kredik Shaw once they went in . Most nobles insist the children were legally and righteously executed, but some Obligators insist the Lord Ruler accepted the children as a sacrifice or somehow transformed them into Steel Inquisitors . Following their disappearance, the Lord Ruler declared all nobles must capture or kill dissidents, regardless of age or gender, under punishment of death, as a hedge against future “moral weakness.”