Anonymous tip (from pay phone) to the police?  I know, not a very earth shattering idea, but it's hard.  Maybe share the link so we can all stay the hell away too!

If you're worried about using your computer, hit the library or something and use the NCMEC websites cyber tipline.  There is also an 800 number.

then I suspect the odds are good you are already talking to the police, you just don't realize it.

What percent of the pimp population even knows what direction that arrow is supposed to point?  It could be they are trying to build in some "plausible deniablity".  

The well being of underage girls are endangered which is much more serious then getting you on some puny soliciting charge.

Try calling 911, let the dispatcher know you need to remain anonymous and share as much info as you can.

You can also look for an anonymous tip line on the LE website or fill out a form online but the process between receipt and action is time consuming and might not be helpful for the time sensitive nature of this case.