In my opinion, this is a very difficult photo, and one that cannot be fixed only with curves -- this photo was probably a difficult image from the minute it was shot due to the dappled sunlight/shade combination hitting the childrens' faces.
You already have experience with retouching, so is your question specifically related to learning to use the Curves function? I'm still not comfortable at using Curves, and have been practicing to get better also. I seriously doubt that Curves could fix most of what is wrong with this image (could be wrong, since I'm not good at Curves yet
You can use Curves or Levels to increase the contrast, but you'll still have to do a lot of individual work on each face to bring out any detail that remains. ONE technique (there is usually more than one way to do something in PShop or PSP) you can try is to add a layer above the working image. then go to Edit-> Fill and choose 50% Gray from the options at the top, make sure that the "preserve Transparency" box at the bottom is NOT checked. and click OK. (Set the Blending mode to Overlay for the Gray layer.) Then use both the Dodge and Burn tools (at lower opacity settings) on the Gray layer to lighten or eliminate some of the shadows on their faces and to burn in some lost detail. The sun spotted areas on the right and left sides could be worked on in similar fashion.