There will be many different ways of achiving this, but this is the way I would go about it. If you have limited time available for the shoot, then practice first.
Mounting the camera
Take a square of scrap wood, and mount a simple tripod head to it in the centre - this will get your camera low. The bigger your piece of wood, the more stable the camera will be, and the flatter it will be. I'd go for something about 50cm square. This will also give your kids a target.
Only some tripods will go low enough for this shot, and they would probably require the legs to spread far enough to interfere with the legs of the players. A tripod would be less easy to level on rough ground as well. Lying under the scrum will raise the camera quite a bit and raises your risk of getting stood on.
Triggering the shot
You'll want a cable/remote release, trigger many many shots, and then filter after the shoot. You'll want to be close to the action yourself, as you'll want to check that the kids are central over the camera.
Posing the shot
Get the kids to try combinations of looking at the camera, and looking up at each other. The pose required may look nothing like a scrum from the outside.
Find a way to make it fun for the kids, especially if they're shy about having their photo taken, you'll get a much more 'camera time' and you'll get much better photos from it.
You'll probably want a medium-small aperature, as you'll want their whole body in focus. Manual focus may be a good idea to avoid the camera focussing on the sky accidentally. Shutter speed doesn't need to be too fast, and as you'll be using a flash, you'll have to keep it below your sync speed.
Start by exposing for the sky - if you have a dull sky this will ruin the image.
Get a flash or two mounted to your board, and find a way to get the light diffused a little - probably push them away from the centre of the board where your camera is mounted. Modify your flash strength to expose the faces to match the sky.
Dealing with Kids
Show them what you're aiming for to start with, provide them feedback by showing them the pictures, and make it fun. If it's a game, kids will maintain their interest a lot longer.
Take some "muck around" shots, this allows them to have fun, without ruining your shot (these sometimes turn out to be the best shots of kids)
- If you can tether your camera to a laptop, it may make it easier to be dynamic with the shots, and provide quicker feedback.
- Be prepared to lie on the floor, and for your gear to get a little dirty.
- If you're doing this at a practice session your choice of 'time of day' may be a little limited, but think about what the sun will be doing, if the sun is high, then it will make it hard to get interesting skies.