Allow me to elaborate on gn0sis's writeup above.
Never use this phrase in Japan. In fact, skip The Three Little Pigs altogether, everywhere in Asia, just to be on the safe side.
This is doubly true if you're teaching English to a group of Japanese 8-year-olds. I'll cut to the chase here. You see, in Japanese, the word "chinchin", or "chinko", is baby-talk for "penis", along the lines of "dick", "willie", "dinkie", or, as gn0sis suggests, "weewee".
Many Japanese people already know the word "hair" as it exists in many product names. So, as the pigs defiantly (and, invariably, nakedly) taunt the wolf, the kids will hear this:
In fact, this is probably what any Japanese adults who are supervising you will hear as well. If you live in a small town, pretty much everyone will now know you as "that foreign guy who told our kids about pubic hair." Your chances of continued employment as an already suspicious English teacher will be even slimmer than before.