Is there anywhere nearby that you could use that is in the shade? Open shade is the best and easiest light to use for portraits. Whatever you do, try to avoid harsh, mid-day sun and shadows. You want to be at least a dozen feet/3.5 metres away from the group. If you can be a bit farther (15-20 feet) it would be better. Choose your lens to comfortably fill the frame from the camera position that works best.
If you can have the group sit on the stairs, you can add to the interest of the image, and more easily avoid the "naval eyed view of the world" that a TLR with a waist level finder tends toward. Try your best to have the camera at or near eye level.
I would use either Plus-X or TMY-2, but those are the films I use anyways. If you are getting a lab to develop and print the results, I would suggest Ilford XP-2.
Can you try a test shot or two?