Don't have them face the sun! Grown adults can't help but squint so kids would be 100000x worse. Personally, when doing portraits I shoot almost into the sun, or it's to the side. The 580EX is a great flash and can compensate the sun (done it thousands of times). For individual shots last year for 5/6 year olds and up I use AV mode and set it to 2.5 - 2.8 - 3.2 (3 shots) using all focus points instead of center point. In bright light, up the flash - at most it's been 2 2/3 I believe. ISO 100 or 200 at most for portraits. The low f/stop isolates the kids and you wouldn't believe how many comments I get from parents like "how did you make little timmy stick out like that, I cant figure it out on my camera?"
Also, have examples ready to show the kids and how they want to pose. I have one standing holding the ball to the side, with there other hand on there hip. One kneeling behind the ball, one handon there hip, the other on the ball or across there knee. One laying on the ground with the ball in front of them, one hand on the ball, the other leaning on a bit, one with one foot on the ball with both hands on there hip (harder to do with younger kids though as there balance isnt there). There's others out there but those are the ones I show them on clipboard and they choose which one they like.
Team pitcure, the coaches will be a lot taller - you can have them kneel down on both ends of the standing row, or just shoot it as is and the coaches will just be "bigger" than them all. I had some coaches that didnt want to get down, they just stood there looking like someone kicked there puppy. 
Hope that helps a little, just my setup but written very fast so hope it all makes sense.