Having trouble charging our iPhones. Is it safe to use a wire brush in the charging port?

I would say absolutely not. If on close inspection with high magnification there was crap in the port I would use a toothpick or stiff toothbrush to clean out the debris. I would also clean the plug coming from the charger in the same way. The contacts should be very shiney. In my experience problems charging almost always are the fault of a bad charger cord. It can happen that the port connection to the charger board on the phone can get damaged but that would require micro soldering or changing out the board depending on the model.