I've seen a FA give a child a sundae once. I figure she was just being nice to the kid. There was also a flight a very very long time ago where the FA had some sort of trivia contest and the winner got a sundae (felt like I was on a WN flight of today).
To the OP, are you certain the sundae is going to a passenger? I almost always see the FA's eating the left overs, so I would assume that they sometimes take it to the back to eat with their meal.
Also, just guessing but maybe if there was an equip change and someone got bumped from F on a transcon flight?