I Saw Mommy Kissing Santa Claus is:

“I Saw Mommy Kissing Santa Claus” was written by Tommie Connor.  The original recording by Jimmy Boyd reached #1 on the Billboard charts in 1952, and on the Cash Box magazine chart at the beginning of the following year. The song was commissioned by Neiman Marcus to promote their Christmas card for the year, which featured an original sketch by artist Perry Barlow, who drew for the New Yorker magazines for many decades.  (Source: Christmas-Cafe)

I saw mommy kissing Santa Claus
Underneath the mistletoe last night.
She didn’t see me creep
Down the stairs to have a peep;
She thought that I was tucked up
in my bedroom fast asleep.

Then, I saw Mommy tickle Santa Claus
Underneath his beard so snowy white;
Oh, what a laugh it would have been
If Daddy had only seen
Mommy kissing Santa Claus last night.

I saw Mommy kissing Santa Claus
Underneath the mistletoe last night.
She didn’t see me creep
Down the stairs to have a peep;
She thought that I was tucked up
in my bedroom fast asleep.

Then, I saw Mommy tickle Santa Claus
Underneath his beard so snowy white;
Oh, what a laugh it would have been
If Daddy had only seen
Mommy kissing Santa Claus last night.