This is my mother-in-law's tried and true trick that she used to get her kids to eat some greens. She said to put celery in, but I thought it made the texture weird, so I leave it out. I think it is pretty good, and Jeanette(who refuses to eat vegetables) doesn't exactly guzzle it, but she drinks it.
Place in the blender:
a handful of spinach
a sprig of parsley
a cup of pineapple juice
Blend until very smooth (you don't want them to detect any bits or chunks!).