Apparantly, they are the children of the trees. The Kodoma appear when a forest is
healthy. When
Ashitaka follows a Kodama through the forest, he talks to it and asks if a tree he passes is its
mother and comments on the
grandeur of the tree.
Also, in the
movie, the Kodamas are the characters with the largest varitey in apperance. Not one Kodama is similar to another.