My restaurant is also loaded with Karaoke singers (some "real vocalists, too) and the easiest method I've found is to use my iPad running Karafun. It's a $10 per month subscription to 40,000 songs, that allows easy-peasy search, singers name, key change, tempo change, and can easily power a second monitor, if your setup can benefit from such. One benefit is that the singer (s) sees a full screen with moving lyrics, and you see the song list and que to continue navigation while they are performing. Awesome easy, and in most cases - fits fright on your keyboard music rack.
Give me a call, if you want more info.