Hmmm... smoothie blends might work for some kids, but I'm unsure if mine would notice the odd flavor. She's very particular about textures - I can see how the creamy spinach blend might just pass in a fruit smoothie! Worth a shot, I suppose, though I'd probably start with a small handful of spinach leaves first, haha.
I've also read about making 'green pancakes' with spinach and bananas, which is a cute idea, and could be fun to whip up in shapes with cookie cutters for a balanced breakfast. Again, it's a texture play and might just work!