Chairs are the most common culprits for walls scrapes. Chair backs frequently bump and slam into smooth drywall, marring its finish and gauging into the surface -- especially when there are children in the mix. Chair-rail molding is a good idea in the long run for areas that see a lot of traffic. Until you get around to a more permant solution -- or the kids grow up -- a simple patch can make your wall look like new again.
Clean the area. You may be able to remove the mark entirely with a wet cloth and soap.
Spread a thin layer of drywall compound over deeper scrapes and gouges. Spread it as smoothly as possible, and don't use more than you need.
Allow the compound to dry completely before adding a second layer, if necessary. Apply the second layer in the same manner as the first.
Smooth over the patched area once it is completely dry by sanding it with a piece of medium- or fine-grit sandpaper.
Paint over the area with paint that matches the wall. If you do not have original paint, you will need to have the paint in the room color-matched at a home improvement store.