I had no idea Yasuko meant peaceful child in Japanese. On the other hand, taking a look at Japanese popular culture, I think we could come across multiple innocent children turned asssassins.
Here is another idea for Yasuko:
The Yasuko Tribal Lands
by HåvardLocation: Davanian Coast
The Yaskuko are the descendants of a noble clan from Ochaela. They were banished from their ancestral lands five centuries ago and fled in disgrace. They vowed to return one day and gain vengance even if their honor could not be restored. Having to find new ways to survive the Yasuko took work as killers for hire, mastering the craft to such a level that they became the ultimate assassins. Today, Assassins employed all over the Known World and the Alphatian Empire secretly hail from the same secret kingdom. Although rumors of such a realm exists, ships sailing along the shores of Davania only report thick jungle and primitive tribal barbarians in the so-called Yasuko Lands. However, the "primitive tribesmen" guard a secret. In their hidden temples is a sophisticated society dedicated to training new generations of Assassins. A Yasuko Assassin is only allowed to return to his lands once his mission has been completed.
Meta: The idea here is to make the Yasuko Lands the home of an Assassin Class, just as Glantri is home to Magic-Users, Rockhome to dwarves etc.