Redwood decking is vulnerable to all sorts of damage because it usually doesn't have a hard finish such as what you find on an interior floor. Your children can scuff it up while playing, and even the pets occasionally have a go at it with their paws or teeth. The fact that the deck doesn't have a finish is an advantage when it comes to removing scratches and damage, but it still takes some effort.
Assess the condition of the deck before you start. If it's discolored or stained and in need of a good power washing, this is the time to do it. Let the wood dry before you sand. If you decide not to clean the deck, you can still sand out the scratches, but the boards you sand may take on a different color than those on the rest of the deck.
Sand out deep cross-grain scratches with a belt sander and 100-grit sandpaper. Hold the sander in the same direction as the wood grain, and press with moderate force. If you sand a portion of a board, continue sanding all the way to the edge to avoid a longitudinal streak on the board.
Work a significant portion of the wood around the scratch rather than simply concentrating of the scratch itself and wearing a depression in the wood. Wear goggles when using a belt sander, keep your hands away from the belt at all times, and read and follow all safety warnings on the machinery.
Go over the parts of the deck you sanded with an orbital sander and 120-grit sandpaper to remove the scratch marks. Use moderate pressure and move the machine along the grain of the wood.
Lighten the pressure on the sander as you approach the ends of the sanded area to feather the edges and avoid transition lines. If you're having trouble avoiding a noticeable color difference between sanded and unsanded areas, you may want to sand the entire length of each scratched board. Wear goggles and keep your fingers away from the sanding pad.
Stain and finish the sanded boards with the same material that's on the rest of the deck. Brush on stain with a paintbrush and wipe it off with a rag. If the deck has a clear varnish finish, brush on a coat with a paintbrush, let it dry, then brush on a second coat.