C.M.
I too love the Deceptively Delicious cookbook by Jessica Seinfeld.
I put pureed cauliflower in mashed potatoes, tuna salad. Pureed butternut squash in mac n' cheese and applesauce muffins, carrots in spaghetti'os. Pureed sweet potato in pancakes, canned pumpkin in oatmeal, etc.
My 3 and 4 year old (and even my husband) don't notice it. In fact my husband, who hates eating veggies too, has said our meals seem to have more flavor to them.
My 4 year old likes to help me cook and I don't keep it a secret that we put extra veggies in the food. She now asks "what all goes in this meal?"
I cook and puree the veggies ahead of time, then pour the puree into ice cube trays, freeze them, and place the cubes in a freezer bag. 4 cubes is approx. 1/2 cup. I can easily grab the amount that I need and quickly thaw them for the meal. One butternut squash will usually fill two ice cube trays which can last me 3-4 weeks, so it's not like I'm constantly cooking and pureeing veggies. I do maybe one veggie once a week. Right now I have bags of cauliflower, sweet potato, carrot, butternut squash, and spinach cubes in the freezer.