The Birds Who Smile: Christmas 2018 - Santa's Sleigh / Reindeer (themes 21 & 19), and also possibly Elf (theme 14) if I can't think of anything better
[rough draft]
A Batman fanfic by Raberba girl
The birds had already had their official visit to the Diamond District Santa Claus the week before, and then there was that unofficial encounter two days ago with the Santa of Burnley Mall, who'd apparently swindled Jack out of $50 (not that anyone had the heart to disillusion the child). Now the Waynes were trying to finish up their shopping at Coventry Mall, making a proper family outing of it with dinner and a movie planned afterward.
Bruce sighed a little when they passed the 'North Pole' and Jack started making an excited fuss, apparently not satisfied with the 1.5 visits to Santa he'd already had this season. Since Bruce Wayne was almost incapable of saying no to small children, cue Santa Claus Visit #2.5. Bruce hauled his youngest child to the back of the line like the non-entitled normal people he was supposed to teach his kids to be, and they waited their turn.
Tim waved and headed off with Cass, Duke, and John to continue shopping; Dick, with Alfred beside him fussing over the shopping bags, collapsed on a bench to rest and strike up a cheerful conversation with the very pretty 20-year-old nursing her baby nearby. Damian was drawn like a magnet to the live reindeer giving rides to children, where he started grilling the attendant about the animal's work conditions.
Peter was bored. Therefore, when no one was looking (except Jason, who pretended he wasn't), he crawled into the pile of fake gifts in Santa's sleigh and lay in wait for an unsuspecting victim.
As soon as the thick velvet cord was lifted up in front of Jack, he raced to climb into Santa's lap.
"Ho ho ho, hello there! What would you like for Chri-?"
"San'a Caus buy chew piggy, rung is happy now?!" Jack inquired eagerly.
While Bruce was explaining to the very confused Mr. Claus, one of the elves moved over to the sleigh to grab the bottle of water she kept hidden in the front.
Peter leaped out in an explosion of fake presents. "HA HA HA HA HA!" he bellowed in his best supervillain laughter, smiling wide like a certain infamous maniac.
Even he was not prepared for the young lady's genuine terror, or the extent of her reaction. "AAAAAAAAAAHHHHHH!" She was already running for her life as Peter, startled, fell back.
"IT'S THE JOKER!" someone in line shrieked, and the entire 'North Pole' erupted into panicked mayhem, which swiftly spread to the rest of the mall. Santa thrust Jack into his father's arms and fled. The abandoned reindeer shifted in alarm and dropped a chunk of poop, but then decided that the treat Damian held under its nose was more interesting than the squealing humans farther away.
The Waynes stood there looking at their kid, the group of them an island of calm amidst the madness. "Pretty impressive, considering the lack of duffel bag heads," Jason remarked in amusement, but Bruce's expression was grim as he comforted Jack.
After a long moment, Peter's rigid, apprehensive pose broke into an artfully sheepish one. With young Tim Drake puppy eyes and a lopsided Dick Grayson smile, he said, "I lub you, Daddy."
Bruce, mostly in an attempt to resist the blatant sucking up, turned away to call Jim Gordon and tell him that the alleged supervillain attack on the mall was a false alarm.