Can You Believe It?

We have a picture book of "Up on the Housetop" and when I read the story to Amelia, I sing it to her instead.  The other day she asked Ken to read it to her, and after a few lines, she said to him, "Daddy, you're doing it wrong".  I told him he had to sing it, instead of read it.  But after his singing attempt, Amelia again told him he was doing it wrong.  I asked, "don't you know that song?".  And he said, "no".  Really?  I learned that song in elementary school and remember singing it every Christmas in school with all the other kids.