Well, the first thing I'd do if I were in your position is start asking people why they don't want me in the pictures :o)

The second is to avoid midday sun at all costs (unless the weather is horribly grey and overcast, which is actually a blessing). If there is open shade large enough for the whole group, use it, otherwise you'll want to have the sun behind them such that there are no harsh highlights or shadows on faces. When you're dealing with little folk, mornings are probably best—they're far less likely to be cranky or filled with wanderlust in the early hours. That may not agree completely with those who are neither very young, very old, or parents of the very young, but they'll just have to suck it up, won't they? There's history to be recorded!

Whatever sort of formation you have the group flying in, it's easier to get everybody in decent focus and fully visible if you elevate the camera a bit (get a couple of steps up on a short stepladder) for the large groups. It puts the plane of focus on a tilt, so that people in the back are at least nearly as sharp as people in the front. This also has the beneficial side-effect of forcing people to look up slightly, doing wonders for things like double chins, turkey necks and Bassett hound jowls without having to resort to individually coaching people out of awkward poses. And it eliminates the middle-of-the-picture horizon line if there's no appreciable backdrop available.

You'll also want to be far enough away that you don't need to use a really wide-angle setting on the lens. Something in the neighborhood of a 35mm-equivalent would be okay (that's as wide as your camera gets, really, but I'm trying to make this geneal) but anything significantly wider than that will begin to visibly distort the people nearest the edges of the frame.

This may also be one of those occasions where you'd want to rent, borrow or steal (only if absolutely necessary) a newer, better camera. 2006 was a long time ago in digital camera years, and even the top-of-the-line pro cameras from that period have trouble keeping up with the better point-and-shoots of the present. Six megapixels from a small, noisy sensor is barely enough, really, for a newspaper-quality 8x10 print. (I know -- the quality seemed magical at the time, but times have changed. What you have is still great for shooting for the web and 4x6 prints, but it's not really "photo quality" for mantlepiece or wall work anymore.) If you can get a recent-vintage camera with an APS-C-sized sensor (or larger) for the day, and preferably for the day before as well (DSLR or mirrorless doesn't really matter, what matters is that you know how to use its basic controls), then you'll be able to create something that's worth framing for posterity.