Hmm, I'm not so sure about the mashed pumpkin idea, but I think grated beetroot and zucchini hidden in a fruity rainbow salad would definitely be a fun and colorful surprise! The sweetness of the fruits could mask the vegetable taste well.
Some other sneakily healthy options which could add some crunch and color include chopping up some capsicum/bell peppers or even some chopped spinach leaves, mixed in with all the fruity goodness.
If you're worried about the veggies overpowersing the fruits' flavors, I suggest going light-handed with the additions and balancing it out by adding a dash of lemon juice and a teaspoon of honey to the salad. It should be a great way to get your little one enjoying some hidden health benefits!