I have never seen anyone check id's. With the set-up of the campfire, you could not even do that. Go and have fun! You can take your own drinks and snacks or buy them there.
Anybody can come to the campfire-movie program. If you have a car park at the reception outpost and take a bus from there to the campfire, otherwise use the Disney Bus system with the appropriate transfers from your hotel at Downtown Disney.
I would guess it is about 1/2 of a mile from the boat dock to the campfire area. Maybe 15 minutes or so walking time. There are plenty of busses you can take to get to the campfire from the marina area. I know both the orange and yellow busses will take you there.