My daughter is 6, and although she is not the least bit picky about food, I've found that V8 Tropical Blends are the easiest thing. You get a full serving of fruits AND veggies in one shot, and they come in a few different flavors. Best of all, your child won't even know it's healthy!!
Watch out, though, because V8 also puts out some "smoothie" drinks and other stuff that aren't actually made of fruits and veggies...just make sure to check the label for the "100% juice" markings.
Hope that helps!