I rented an apartment that was advertised as Montmartre (18th arrondisement) which, while technically was in this district, it bordered on the 9th. Our closest metro stop was Pigalle. There was an interesting establishment next door,complete with lovely lights that came on every afternoon and stayed on until the wee hours, featuring rather unattractive, rough looking women who would stand outside making all the men feel "welcome." Enough said! We walked by every day on our way to the metro, past the sex shops, with interesting windows, etc. My husband still tells people what a great wife he has!!! Lolol!
We are adults and travel with a fun loving couple and it didn't bother us. We never felt in any danger, but I would have absolutely croaked if I had to walk the kids down the street without blindfolds!
Agree, Google Map it! Very unlikely you'll ever get mugged anywhere in Paris, but you do have to keep control of your bags, wallets, and especially iPhones (yes, we had one stolen but not in this neighborhood.
Have fun in Paris, but the 9th is a little rough for my tastes.