Personally I am more than happy to have visitors to the FD before or after the flight. I'm enthusiastic about my job and love it when visitors (young and old) share my enthusiasm.
Pretty much all my colleagues feel the same way - that's why we fly for a living. The only time we'd have to say no is on a very short turnround, when we're up against the clock and usually playing catch up.
Always worth asking the cabin crew.