Tim in MA saw this in a restroom. Perhaps the quotation marks are there because there are no "children" in an adult restroom. Or it's a well-known slogan which I don't know and they are citing without attribution.
I don't usually post the following: newspaper headlines, personal email, craigslist postings, unprofessional websites. I also tend to not crosspost things from other blogs, since I have so much unique material waiting for me to get to it.
Things I see a lot: silica gel "do not eat"; hair dryer labels; inside the bus "do not drill"; Wal-mart sign about IDs; coffee machine with "2" cup sizes; employees must "wash hands".