Preheat oven to 425° and cover a baking sheet with aluminum foil. Arrange sweet potatoes on the baking sheet, then drizzle with 2 tablespoons olive oil and season with salt and pepper. Toss to coat and roast until golden and tender, about 25 minutes.
Meanwhile, combine quinoa and 2 cups of water in a medium saucepan. Bring to a boil, then reduce heat and simmer, covered, 15 minutes. Remove from heat and let sit, covered, 5 minutes more. Scoop quinoa onto a paper towel-lined baking sheet to cool slightly.
In a large bowl, combine quinoa, sweet potatoes, kale, and cranberries. In a small bowl, whisk together balsamic vinegar and remaining 3 tablespoons olive oil. Drizzle vinaigrette over salad, tossing gently to combine. Season with salt and pepper and stir in feta just before serving.