When I hear 'I saw mommy kissing Santa Claus' , I don't picture the Santa Claus in question being the father of the obviously male child spying on his parents having a snog. I think mommy caught the real Santa Claus off guard, and that daddy is sound asleep after a nasty week at the office and a few bourbons before bed.
Since our desperate 1950s housewife hasn't seen the postman in a while due to the holidays, she turned to the first man who drops by and that just happens to be Santa. Santa is the master of sneaking in and out unnoticed, someone who never kisses and tells, and probably has more housewife experience than anyone else on the planet.
Maybe the husband doesn't make enough money and mommy has to use her charms to make sure her little boy gets that train set hubby can't afford. Maybe daddy is always on the sauce and doesn't pay any attention to what his wife does at night. It's all about keeping up appearances and Santa knows all about that, too.
Bonus points for making the little boy think that mommy is either 'a very friendly person' to strangers or that daddy dresses up like Santa at Christmas and that's how he is going to get a baby brother to play with next year.
Once you've conjured up these images, listen to Québec diva, Ginette Reno singing 'Maman embrasse le Père Noël' ('I saw mommy kissing Santa Claus') in country music style.