What does it mean if your child has very green diarrhea?

Normal variation. The color of poo is derived from the waste pigment that end up in the bile, which is added to the intestines from the liver. It starts out an intense green & over time may change thru yellow & brown. The germs of the intestine convert the pigment as it transits the digestive tract. The less germ action the less color change. Diarrhea is often a greenish color for this reason.