I was there in the middle of the day, but I could see that in this rather isolated corner of the park, night time could be a problem. Most probably there had been one or more incidents in this area (there were two signs within 30m of one another, with the warning printed on both sides).
It's an unpleasant topic. But two other articles along the same line came to my attention last week.
The first one was about sexual assault on girls on trains and it is shocking, not just because it goes on, but because many girls don't realise that being fondled etc. is abuse.
The second is titled "Japan child abise, pornography cases hit record highs in 2016". Again, tragic.
Not a nice topic at all, but it's good to be aware and is definitely worthy as fuel for our prayers.