In the very last few lines of the story, the narrator justifies the title:
Of all who give gifts, these two were the most wise. Of all who give and receive gifts, such as they are the most wise. Everywhere they are the wise ones. They are the magi.
He has said that the original magi were "wonderfully wise men," so it stands to reason that their gifts would be wise gifts as well. The "two children" of this story are not wise, at least in the typical sense of the word, because they sold their most prized possessions in order to purchase a gift for the other. In a way, it is unwise to part with the object dearest to you. However, these "children" are even more wise than the magi because they had to realize something the magi did not: that the person they love is far more precious than any object, however meaningful, could ever be. The magi were kings and did not have to make a personal sacrifice in order to express their love to the Christ child; as rich kings, they could easily afford whatever expensive gifts they chose to offer. Jim and Della, on the other hand, each made a personal sacrifice, giving up the only thing each had of any value in order to show their love. This makes them even wiser than the original wise men; Jim and Della, then, are the true magi.