After reading your gear:
1) buy some reflective umbrellas. They're dirt cheap.
2) but the $30 Cowboy studios radio triggers from amazon.
3) stands? I'd guess you have stands? Do you have a mount that can hold your umbrella and flash?
4) gels. You'll need some gels for your speedlights if you have ambient in the background that is hitting the ice. What they call CTO, or orange looking.
Your two speedlights are perfectly sufficient with this setup.
You're working with a 50mm, so your choices are going to be a bit limited as far as getting a blurred background if you decide to shoot inside the arena. Do you not have an 80-200 f/2.8? That's a great lens from arena background shots.
You want your umbrellas at about a 45 degree angle from your subject, and as close as you can get them without them intruding into the actual shot. I usually find that 5.5 feet in front, and 5 feet apart, with your subject dead centered between them 'just works' It's not the most dramatic light, but you don't have to fiddle with it, and you can see the kids face, and you don't have to worry about shadows. Parents like it. It looks traditional.
CHECK YOUR SHARPNESS BY ZOOMING EVERY SINGLE SHOT, ALWAYS AND FOREVER. Do not let a kid get up until you zoom into the eyes and are happy.
Athletes look best either kneeling or standing. Sitting makes them look unathletic.
I'd probably have them stand, tell them to turn their shoulders and feet towards one of the umbrellas, and then turn their head straight at the camera. For Hockey you can have their hands on top of the stick. 3/4 shot on a 50mm looks good usually.
Settings wise, if you use the arena as a background, you need f/2.8 and at least 70mm, unless the arena is cavernous. Get as far away from your background as feasible, to give the longest throw. If you go with the 50mm, still use f.2.8, because while you want the background blurred, you need at least f/2.8 so that the subject will be fully in focus.
If you're shooting up against a close background and aren't blurring the background, or are using a backdrop, shoot at f/11.
Shoot in manual mode. ISO 200 is usually where I start, just so I have a stop down I can go quickly if I need to. We already discussed aperture.
Im guessing you don't have a light meter. You'll have to eyeball it with test shots.
Use your shutter speed to get your background like you want it (unless again you are shooting with a backdrop). Then use your flash power to get your subject exposed like you want. (make sure you have somebody or some thing to stand in for a subject. I usually start with 1/4 power and work from there, playing the brighter/darker game.
Remember, ISO and aperture control global exposure, ie subject and background. Shutter speed controls only background and flash power and distance controls only subject exposure.
edit: seems like this may be more trouble than you're willing to go through with? Find an actual pro to do the reshoot then? I'll leave the above up, for the reference of others.
