Quite often if we have long slots then it's common to let the kids up to visit the flight deck. It keeps the parents happy, is good for
PR and may inspire the next batch of new pilots. It was visits to the flight deck as a kid that got me hooked.....!!
Officially haven't seen it written down anywhere that pre / post flights are not permitted although there is an arguement for post flight only due to the pre flight, flight deck security checks ..........