Personally, I think we should never let our children believed that the gifts they recieved during the Christmas morning comes from a magical man called Santa Claus, but rather teach them to appreciate any gift we parent had been sacrificing to provide. It is simply because we might entail to our kids mind about greed and materialism which may overshadow the Christmas season in which the true meaning would be about giving, loving, and worshiping God. If a child who belongs to a poor family, they might be overlooked by Santa during Christmas. And much worse, be treated as "naughty" boys or girls.
As parents, our kids trusted us to tell the truth. Its our responsibility as parents not to break this trust. Once broken, we may not able to convince our kids about the more important truth about Christ, which our kids are not able to physically see.