"The child is in the class."
Translation:Đứa trẻ đang ở trong lớp học.
Is this a special case where you can use 'ở' and 'trong' at the same time? Seems redundant.
No, these two words are actually quite frequently used together. In Vietnamese, two synonyms are in some cases used together.
Would you use in for cases like "nhà", "toà nhà", nhà hàng, khách sạn, etc?