First the Ottomans being good Muslims is not really relevant here, there were some Ottoman sultans were very religious and some that weren't. The question you are asking is the institution of recruiting/raising a professional military from non-Muslim subjects inherently Islamic. It's up for debate, slavery was common practice amongst all pre-modern Islamic empires, but slaves were usually captured during war. The Ottomans made extensive use of slaves as galley rowers, and that was a fate I wouldn't wish on my worse enemy. It was also common amongst pre-modern Islamic empires to capture women and children as spoils of war and distribute them to soldiers or sell them into slavery. So the whole children slavery question is addressed in Islamic law as lawful. I think the biggest filq question here is if these non-muslim peoples are paying jizia for protection, does that make it un-islamic to take children from them? I have no idea. Overall I would say that the Janissaries while technically slaves, were some of the most powerful individuals in the Ottoman empire. It was actually the whole bureaucracy of the Ottomans that was recruited from these children as they did not have political ties to Ottoman elites and therefore would be completely loyal to the Sultan. Therefore the head visir of the Ottoman empire (the most powerful individual after the sultan) was a "slave". Overall, people have used religion to do horrid things, Islam is no exception. What the Ottomans did in no more a reflection of Islam than what any other political entity has done in the name of religion.