We also lied to our son but as he soon learned it was mum and dad as all kids with older brothers in school would tell them.
as a kid, I was told that some people believe in it and it isn't my place to tell them the truth - they will find out soon enough.
In Spain, the tradition was to give presents on the Three Kings Day, which is January the 6th.
The Epiphany is a holiday here too, but not a gift giving day. The 13th of Jan is an interesting one as in some small areas, the tradition is to knock on the neighbors door and ask for something to drink :)
It was funny to hear he recognized my wife's writing in the letter as he was growing up.
this is part of the fun of it for parents perhaps. =)