There's no reason to be scared unless you try to kill or rob someone in front of them. Cops, like any other worker, don't like to work on their lunch breaks so they are not going to come to the restaurant and actively look to questioning employees about their immigration status.

The cops are there to eat, so just let them eat in peace and be friendly, thats all. If you live in NYC, then you're even more better off because the NYPD here just doesn't give a crap about a guy working hard in the kitchen and staying honest, who just may be here illegally. Our city has got bigger problems.