They are children, even if they have Hunter licenses, and they aren't responsible for an entire kingdom's safety. Optimism and belief in the better nature of others is for fairy tales, not prolonged wars with an ever-renewing enemy where every inch of ground gained comes at the cost of thousands of lives. He does not have Ozpin's faith in any souls, simple or otherwise, and so when those children work against him, he will treat them as his enemies.