Most of the side notes I could add about this event are not related to photography, rather organization and showing the results. The main reason is that each event is special, has its own lighting and shooting conditions. Here, the main factor that struck me at first was the mix of light and harsh shadows coming through the old oaks foliage. That made me think of excellent rendition of this lighting by impressionist painters. Unfortunately, this is a difficult lighting for people groups, and many photographers got bad results. Either by inadequate exposure (generally underexposure) or harsh shadows on faces.
http://www.prestophoto.com/photos/image/1702469/43263
(out of camera jpeg)
My first choice was to use the onboard flash of my 20D for fill in. The results seemed good on my display and I saw I could trust the automatic settings without any compensation. My second choice was to prefer backlighting or shadow for small group shots, or shooting from slightly above. This led me to prefer an external flash, even if this makes myself very conspicuous. Generally, I got decent results, and other DSLR users followed my example successfully.
When we shot the scenes of children bathing in the water in full daylight, I kept the flash for fill in (a more powerful unit would have been still better when subjects were more than 5 or 6 meters away.
As already mentionned, I also used the fill in for the big group shots.
One of the result of this mixed lighting was that I had to compensate in editing for the green or cold color casts, either' by changing the color temp. balance (for my raw files) and/or using the 'correct color for skin tones' feature or the warming photo filter.