While etching ink is the best option for altering the coloration and adding an overcoat to oil pastels, sometimes the turpentine is a bit worrisome to some parents. So, for a beginner use a diluted mix of acrylic to stain the oil, and then begin etching.
1.) Draw your subject matter using oil pastel. Pay close attention to shadow, value, and color key.
2.) Apply a colored ink (or acrylic stain in this case) that complements or brings out the key of the work. If you're working with a sunset, oranges, red, or twilight mauve purples work well. If you're working on a night-time scene go for a payne's grey hue. If you want an arctic glacier or underwater dreamy scene you could use indigo or aquatic blue.
3.) Apply the stain and let it dry. Go in with wood-carving tools or toothpicks for young ones. Etch and remove the overcoat to reveal the brightly colored paper beneath.
This technique is a lot like a scratchboard, but you can interact with layers with a lot more say. There trick with this technique is interlocking the etching with the undercoat achieved by applying an overcoat stain.
Check out Susan Seddon Boulet's life's work for some GREAT examples. Remember, if you can't use the turpentine, just go for the acrylic stain. You won't get the same subtleties, but it's great for a beginner.