Parental love is a very complicated emotion.
If the parent is looking after the child's best interests, then arguably their actions reflect love. While probably an exaggeration, there is still a bit of truth here. One can usually find a kernel of love towards their children in even the most narcissistic of parents. Furthermore, the tears a narcissistic parent sheds when their child dies are absolutely real. Simply put, love is too complicated an emotion to be of much use in distinguishing narcissistic and healthy parents.