You were L's child. If he had to go somewhere, you were with him. Because of the Kira case, you were staying at the hotel with him, usually also sticking around for meetings. As a result of this, you became friends with Light, as you were relatively close in age. The two of you were actually good friends. Despite the fact he despised L, he found your presence more bearable, not to mention convenient to his plans. If he could become close with you, taking out L would be significantly easier.