I have two nine week old kittens and have never had kittens before and wondered is green stools something to worry about

I wouldn't worry if stool is still green by the time it is excreted, it could mean that it went through the large intestine too fast to be changed in color. This is often called "rapid transit" or "decreased colonic transit time". It is a normal occurrence in young mammals.Dont necessarily look for brown stools too often until they are developed a little more and eating regular cat food.