The shuttle is a jitney so you need to be able to get in the back - I think if you contact Speedy's you might be able to arrange a taxi from the dock - just a side note the ferry is not that easy to board and un-board (narrow gangway) in any event (they are temporarily using smiths old ferries operating under Speedy's name) - I'm sure you can temporarily dock at trellis if that is an easier option. You might want to consider taking the ferry from VG to Road Town then taking a bigger taxi which will deliver you to the front door of the air terminal or depending on mobility just walk from the trellis bay dock to the airport (the jitney goes around to the front door) which is not how the crow flies nor how you would walk to the airport from trellis - as long as your parents don't have a lot of luggage or you manage it for them the walk wouldn't be difficult. cheers Brian