In a gym, I would just shoot the flash right at them...because bouncing off the ceiling probably won't do a whole lot. The Fong Dong is at it's best when bouncing off of walls and ceiling...so I think that it would be a waste in a gym.
One concern will be the light temp of the lights in the gym...they probably won't mix well with your flash. You could try to overpower the ambient with your flash but that might leave the background looking very yellow(ish). Another option would be to gel your flash to match the lights.
From what I've seen of these types of photos...I think that simple lighting is best. If you can away with just using your flash, the I'd go with that. You should have much of a problem with shadows, if it's on a bracket...but I'd be careful of putting them up against a wall anyway.
If you can get two lights and have them off camera, that might be best for the group shot...but the one flash may be good enough. Besides, these types of shots are practically 'documentary' type shots anyway.
The individual shots can be more creative but I still think the lighting should be kept fairly simple. (these are young kids after all). If you have a wall that is not too colored, maybe move them close to it and bounce the flash right off the wall.
Maybe get creative with poses and props (ball, hoop, shoes etc). I think that if you get a shot that is sharp, well exposed and has a good expression...it will be successful. Add a prop or two and the parents will be very happy.