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...Will I get away with the flash units or should I rent some alienbees?. Never shot this before. Any tips you might have would be appreciated.
The Speedlites should be fine.
For the individual shots I'd use one of the big ones for bounce light for broad fill (a back wall, if available, or the ceiling), one for your key light (centered above the camera, probably), and one for a rim light (behind and to one side of the subjects).
For the group shots, turn off the rim light. (It'll be too close to one side of the group, too far from the other.) If you have time, moving the rim light to act as a second key light would help with the large group.
If any of the shots are "action shots" with the dancers in motion, use the fastest shutter speed you can sync with (1/200 on the Mark III, I believe), with a high enough ISO setting and wide enough aperture to get the correct exposure with the light produced by your Speedlites, for sharp shots. Using a slower shutter with rear-curtain sync will yield more-blurrred shots with a crisp finish, for a more artistic rendering. If you can't do both, then I'd suggest just the sharp ones, because most parents will want to see their kids "properly focussed."
Addendum: With the non-Canon radio triggers you may not be able to sync at 1/200; you may be limited to 1/125 or even 1/60. Practice with your camera and your radios before the event to determine the max sync you can get.