Beach Family Portraits with borrowed equipment

[dainwaris]

I'm a fairly experienced hobby photographer, with good control of a camera, but have little experience in organized portraiture. Close candids of people in ambient light I do well, but I'm being asked to stretch a bit next week.

My extended family is taking a vacation to California all together. We'd like to memorialize the time together, and that will mean a picture of all 19 of us, including children ages 1, 1, 2, 2, 4, 4, 7, 8, 8. We'll be near the beach, and that was mentioned for setting, but we could avoid that if it heightens the difficulty.

The equipment I'll be using is mostly pieced together from stuff we own, and stuff borrowed. I'll be shooting with the following, which is exciting enough for me, as I'm used to my old Rebel Xti and 50mm 1.8:

Canon 5D Mark II
430 ex Speedlite
24-70mm 2.4L
good tripod
large silver reflector
wireless shutter release

Like I said, with this setup, how can I go wrong? But that's my worry, to squander this confluence of events.

Any advice out there on a couple of issues?:

Lighting: Any thoughts about shooting in beach lighting using the reflector and speedlite as fill? Am I going to be too far away to use these effectively? Early morning light or the more directional pre-sundown light?

Settings: I'm assuming I'll need to shoot with my f-stop around 8. What focal length will I likely need to use with this many people, and this lens? It'll be my first use of the 5D mark II. Any capabilities or settings I should know of that may be useful here?

Dealing with this many people: I'm afraid posing will be rather ad hoc, but any advice on this? I'll be shooting with a wireless shutter release for the first time, too, since I'll be in the pic.

Like I said, I've got good command of my Rebel in Av mode, but this seems like pinch hitting in the majors for one at-bat.