Mr Incongnito,
I suggest you tell them to request a flight-deck visit during the flight via a member of the cabin-crew. Most captains are more than happy to accept visitors as long as it is not a training flight. As for the jump-seat, that is totally up to the operating skipper on the day and probably the best advice is to wait to be asked rather than asking out-right.
Hope this helps.
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"After V1, we'll take the emergency into the air - You call 'Positive Climb' and I'll ask for the gear up. We'll climb away at V2 +15"