With respect...
..it is nothing to do with Flight Crews children disconnecting AP's - although this was a major problem for any number of other reasons.
The locked FD is a direct result of the Sept 2001 attacks, and the following knee jerk reactions. Whether it is right or wrong has been done to death on PPRuNe before.
However, if you were on a flight I was operating, then as long as the Skipper was okay with it (and 90% of them are) then I wouldn't have any problem with you visiting before or after the flight - obviously on the ground with engines cut off. And it would depend on it not being a busy / critical time for us.
I've not been flying long, but I love it when we have visitors, and most of the Captains I fly with also do. If I was you, I would politely ask a member of Cabin Crew when you are boarding and see what they say - they can only say no.
Hope you get up there!
DW.