Moxie confirms that you can't taste spinach when it's blended in a smoothie (she includes some nice looking recipes, too). My kids would view a green drink with suspicion, but perhaps yours won't?
UPDATE: My son is a huge fan of the Greensicle, a kale-plus-fruit smoothie sold at Sip. He balked at first, but when I refused to buy anything else, he went for it. And loved it.
My attempts to make green smoothies at home haven't been as successful, but that's because my blender doesn't puree everything smoothly. Tiny bits of chopped leaf = yuck. Sip employees and several friends swear by the Vitamix blender (to the point of rabid passion), but I can't bring myself to spend that kind of money.
Any other blender recommendations?
Related: Baby food fruit sweetens smoothies