Combine lime juice, scallion whites, and serranos in large bowl and let sit for 5 minutes. Stir in sugar, if using, and salt. Add watermelon, ½ cup cotija, ¼ cup cilantro, ¼ cup pepitas, and scallion greens and stir to combine. Transfer to shallow serving bowl. Sprinkle with remaining ¼ cup cotija, remaining 1 tablespoon cilantro, and remaining 1 tablespoon pepitas and serve.