Hm. My gut tells me that wider is better here. In this case I'd grab something as wide as 16/17mm or even the 24 as Ed suggests to complement the 50mm. I think wider is better because although a long lens will give you better and obviously closer portraits, I suspect there will be so much action that most of your shots should be including multiple people, showing dynamics between them etc.
Be careful not to play too passive and shoot from the outside too much if you are shooting wide to 50mm. Get in there !
Edit: in terms of lighting, a backyard shoot (assuming no e.g. tent ceiling) with just one flash is a challenge depending on how you look at it. If you're cool with subject in a coalmine shots that have flat lighting on the subject's face, well, that's easy: pop the flash on your camera. If you want nicer lighting, put it off-axis, but prepare for serious serious challenges in getting keepers for multiple reasons:
1) shadows created due to occlusion (remedy: put flash up high, walk to and shoot subjects you KNOW are getting the light on them.
2) variance in intensity of light falling on subjects (remedy: increasing distance of flash from subjects makes for more even light)
3) hard to grab focus in the dark (remedy: use a flash with flash assist on your camera, put wireless through output on side of camera (pc sync port ?)
4) light falling at bad angle aka, half-lit face (remedy: wait until subject turns to face best angle or look for subject already doing that. OR move flash)
The above 4 points should give you a sense of the difficulties of going wireless with a flash outside with no (ceiling) bounce sources. Should you take on the challenge, as always, set the ambient for the background, ie., you'll likely be at ISO 1600+.
