Why are Jim and Delia called two foolish children? Why are they said to have been the wisest of those who exchange gifts?

Many would argue that they were incredibly foolish to give
up that which was most precious to them.  Delia loved her hair and it was renowned as
being quite beautiful.  Jim was very proud of his watch and it had been handed down
through his family so it wasn't something to be sold lightly.  So on the surface, their
exchange was foolish as they ended up with, practically speaking, useless
gifts.

Of course the wisdom of their exchange was that it
showed such great love for the recipient of the gift.  A young couple, struggling to
make ends meet sacrifice everything for each other, likely demonstrating a commitment
and willingness to sacrifice that will see them through any kind of tough times
together.  Perhaps this is why they are later said to have been so
wise.