A while back I bought a book called Deceptively Delicious by Jessica Seinfeld. It is all about adding pureed veggies to your food to make it extra nutritious with out your kids even knowing that they ate veggies. You are still supposed to offer the veggies in their whole form too, this just ensures that they get the benefits from them, whether or not they eat them. I don't completely follow her recipes, but I use them as inspiration to figure out which purees to add to certain foods. I have had a lot of people asking me how to do it. It could not be easier!
First, you chunk up your veggies and place them in a glass bowl or steamer. I read something a long time ago that said you should keep your veggies in big chunks when you cook them. Less surface area means less "lost nutrition" from the cooking process. I don't know if it's true, but it makes sense so I do it that way!
Place cooked veggies in a food processor or blender and add a small amount of water. I like to use my cooking liquid so that I can get back any nutrients that cooked out. Puree until smooth, adding more water as needed.