Greenish coloured stools, also so-called "starvation stools," could be an indicator that a child is not being fed well; that he is starving.
This is so as, suboptimal feeding makes small volumes of food materials to be present in the intestine. Most of these would be absorbed by the body, and what would be present at the distal end of the intestine would just be bile, which would then run through, like water in a pipe, without enough time for intestinal flora to convert Bilirubin to Stercobilin. It would then come out as greenish coloured watery stools.