A young child will be impressed by either park. For the parents sake, I would visit Sea World if you only have one day and you will be visiting WDW at other times in the near future (and have in the near past). Sea World is a beautiful park, and it's peacefulness is a much appreciated change of pace from the excitement of Disney parks. I consider the Magic Kingdom exciting and enchanting while Sea World is relaxing and breathtaking. I think the beautiful and relaxing atmosphere of Sea World will leave just as important of an impression on the development of a young person as the excitment and thrill of the Magic Kingdom, so spend a day at Sea World, and allow her, and yourself, to experience both.