At the end of the story, O.Henry seems to be saying two different things. Read below from page 6 in The Gift of the Magi.

And here I have told you the story of two children who were not wise. Each sold the most valuable thing he owned in order to buy a gift for the other. But let me speak a last word to the wise of these days: Of all who give gifts, these two were the most wise.

Who are the two children? Jim and Della

Why does O' Henry refer to them as "children"? Although they are both young adults, they are simple like children in many ways.

How were they unwise? They sold valuable things to buy gifts that they didn't really need to buy in the first place, because they were truly in love.

How were they wise? They thought only of the other person and not of themselves when they gave their gifts.