Kumamoto is generally a safe place. My kids had to pass through the red light district (yukaku, 遊郭 - it extends south of Shimotori to Shirakawa, and not coincidentally, the far side of the river is studded with love hotels) on their way to and from juku and never experienced any problems. The area around Kamitori/Shimotori is called “hankagai” (繁華街), meaning a commercial/entertainment area. (When my kids were young and I asked them where they were going, they answered “町” machi, downtown - in Kumamoto, this has the same meaning as 繁華街).
But some dubious activities do occur, mostly involving touts (kyakuhiki, 客引き) who lure generally previously inebriated customers into dens of ill repute - known as fuyoku (風俗）- entertainment and amusement trades such as bars, restaurants, mah‐jong parlors - and fleece them. Kumanichi reports that a new prefectural ordinance has begun which prohibits such activity in the 繁華街 area, though distribution of flyers and tissue is still allowed - Good thing: I like free tissue. - William