As O. Henry confirms, the magi referred to in The Gift of the Magi were "wonderfully wise men who brought gifts to the Babe in the manger." Henry is creating comparisons between the long ago Biblical events surrounding that series of gifts and the gifts Jim and Della gave to each other in relatively modern times.
The magi gave gifts not specifically identified in the story but undoubtedly carefully chosen to be "wise ones" - which Henry wants us to interpret as meaning valuable, practical, and, in a comment intended to reflect modern gift-giving strategies, capable of being exchanged if the gifts were not needed or had already been received from another source.
Jim and Della were "two foolish children" in the sense of buying gifts they could not afford for each other at great price - "most unwisely sacrificed for each other the greatest treasures of their house." But in a deeper sense, they were the wisest of all gift givers; they gave the gift of love, that most valuable of all gifts that is practical and never needs to be returned or exchanged.