My kids are really very cheerful about eating vegetables, it must be said. There was no magic trick that we performed, aside from a) serving them a wide variety of vegetables all through their lives and b) eating a wide variety of vegetables ourselves. So all three of my kids were very enthusiastic about this soup, while your kids might not be. It's a VERY yummy soup - flavourful and creamy and good - so perhaps your munchkins will love it, too.
Heat up some olive oil in a largish saucepan. Chop up an onion and cook until soft in the oil. Add:
1 head of chopped cauliflower
2 cups chicken or vegetable broth
2 cups milk (I used a bit of cream as well that I had in the fridge and it was very nice)
And that's it! Let it simmer until it's tender, which should take about 15 minutes. Now you're going to pureer it in batches in your blender or with an immersion blender. Add salt and pepper to taste and serve. You can grate some Cheddar on top if you like.