I will never say No to someone that wants to visit the cockpit after a flight, specially if it´s a kid or a student pilot, unless we have any good reason like a very short turn around or a technical issue to solve.
Many people love planes and aviation but they are not as lucky as us to be in a cockpit everyday, so I´m really happy to help them and show them the cockpit, for many of them is like a dream to visit us and take a few pics with pilots and in the cockpit, even very famous guys i have had in the cockpit and they have been very nice and thankful for the opportunity given.
I have even had other pilots in the cockpit during flight and the same, they have been very thankful for the ride and the opportunity to take pictures of places that they were going on holidays.