Ah! That is because it's in the Accusative case . . . probably! If you turn the question into a statement, then Orangensaft is a direct object, so you need to decline Welche just as you would decline "der." Right?
You have explained it in such a simple way..thank you.. could you please help me understand nominative,accusative, dative and genitive cases too..I am not able to get them how much ever I am banging my head
Although this should be taken into account, we should be learning German through our best English: "What Child?" is not as good as "which child?" and besides, "which" sounds more natural in the context