I'm certainly not an attorney and my reply is only my opinion based on life's observations, but it would seem to me that when the gun comes out of the holster, ALL liability would be in your hands. Since there appears to be no set gun "policy" in place and what appears to be a great number of customers in your store (based on your original posting), a "Negligent Discharge", with injury, may get the lawyers and your insurance company busy in this sue happy society we live in over this imagined incident. And guess who allowed this practice and eventual discharge to happen?
I support all gun stores that allow both open and CC carry and would like to see ALL stores apply this common sense practice, it is a dad gum gun store! But I do understand some of the reasoning behind the no-gun, no-loaded gun policies especially when they have to deal with their insurance companies demands, company costs including bonds and possibly state and city laws.
The one thing that a store has NO control over is the "Customers Level of Training". In time, their level is revealed in either their conversations, how they handle weapons you are showing them for purchase or their range activities. It may be enlightening for you and your father/son to take some time with your customers and quiz them on their firearms background and maybe a deeper communication with your insurance company and your business attorney in order to make a better informed decision as to whether you should put up a sign or not.