I like cauliflower just fine...but I can't convince myself they're potatoes! However, cooked turnips seem more like potatoes to me. Others have posted that they've roasted or boiled radishes and mashed them and come up with a believable mashed potato concoction.

For picky eaters, you can always keep a bag of pre-washed salad mix in the fridge...then if he doesn't like that night's new cooked veggie recipe, you have an alternative to serve!