User blog comment:B214/Chapter 510: Natsu's Heart/@comment-7418318-20161114111629/@comment-27281341-20161114114445

If you remember, Igneel had a conversation with Grandeeney, after the Tower of Heaven arc and Grandeeney told Igneel that she hoped Wendy and Natsu would make good friends. So if think that "that child" is probably Natsu.