Most people who have been sorting have found a few of these "silver coated" pennies, usually with dates from the 1960's. They are more than likely Mercury coated and while it won't kill you in small quantity, it should be noted that this stuff is not good to handle. It is poison, especially in small children.
I know some people think mercury is harmless. Feel free to break open a thermometer and handle the stuff, put it close to your nose and inhale deeply. It won't hurt you. Here is a youtube video I found on how to make your own Mercury coated penny! For the kids or grandkids to play with, you know...like WE did? Pass on this tradition! It won't harm your kids at all if they put it in their mouths either. Oh and breath DEEPLY when doing this experiment with the kids close by also. Enjoy!