Preheat the oven to 450 degrees . In a small saucepan, bring the carrot and pineapple juices, half of the shallots, the sugar and crushed red pepper to a boil, stirring. Reduce the heat to medium and simmer, stirring occasionally, until the sauce is thick and reduced to about 1/3 cup, 20 to 25 minutes. Strain and discard the solids; season with salt and pepper.
Meanwhile, place the cauliflower on a baking sheet and toss with the olive oil, salt and pepper; spread evenly and roast for 10 minutes. Add the remaining shallots and toss, then roast until the cauliflower is browned and just tender, 10 to 12 minutes.
Toss the cauliflower with three-quarters of the sauce and half of the parsley and pumpkin seeds. To serve, drizzle with the remaining sauce and top with the remaining parsley and seeds.