Make mashed potatoes healthier by poaching them in low fat milk and mashing cauliflower together with the potatoes.
Add 2 T canned pumpkin per serving to pasta sauce. They won't notice and it's a full serving of fruit/veg.
Roasted butternut squash or beets in very small chunks can be added to ANY sauce to punch up nutrition. It's a lightly sweet taste that kids love if they don't know what it is.
Pickle it. 2 T rice wine or coconut vinegar, 1 t sugar, 1-2 T water. Have the kids slice assorted veggies and toss in the pickling mixture. Man...they gobble this stuff up.