We are flying from Nuremberg, Germany to our connecting flight in Frankfurt, Germany to Seattle. Will we have to go through customs/ security in Frankfurt? It's just an hour layover!
You do not go through customs in Frankfurt. You will go through exit immigration as you are leaving Schengen. Your passport has to be stamped.
Customs is for goods. Immigration is for people and passports. It's not one thing.
Before reaching the gate to SEA you will go through passport control and also (separately) through the metal detectors for the security check. If you fly Lufthansa the US bound flights will be in the Z gates, which is in the same terminal as your arrival flight from Nuremberg, just above on the next story up.