How to report anonymously someone watching child pornography?

Next time, just plug your question into the search bar...
https://search.yahoo.com/search?p=how+to+report+child+pornography&fr=yfp-t&fp=1&toggle=1&cop=mss&ei=UTF-8
The department of justice handles these sorts of things due to the interstate nature of the crimes.

police tip line from a burner cell phone

Your local police department should have some sort of tip hotline where you can report anonymously.

Local crime stoppers