This question was closed as "Unclear what you are asking". The question itself seems pretty clear to me. The OP wants to know why did Eddard have to take Arya with him to King's Landing. Sansa's case is clear, she was betrothed to the Crown Prince and it would have helped her to know him better if she lived close enough and she'd also be able to learn about her duties as the future Queen by living at the Court. Arya's case can understandably elude some viewers. The already-existing answers are pretty good too.
As stated already, it seems clear enough to me as it is but in case that I am wrong, how can we make it clearer?