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.