This is a bonus flashback scene from Robb and Jon's early childhood.
EXT. WINTERFELL COURTYARD - DAY
Snow falls thick and soft from the sky.
The courtyard is covered in a foot or more of snow. In the protected spaces under roofs and walkways, servants are busy at their work.
A snowball flies through the air and splatters itself against a snow fort. A head of curly brown hair pops up from inside the fort. It's TINY ROBB STARK. He's about five or six years old. He brandishes a snowball of his own.
Who goes there!
TINY JON SNOW appears, brandishing another snowball.
It is I, the King Beyond the Wall! Come to take back my land!
Robb spits scornfully in the snow, though he doesn't quite do it right and ends up with spittle on his chin.
We know no king but the King in the North, and his name is me!
Robb throws his snowball, which Jon dodges. A flurry of snowballs criss-cross the air as they laugh and duck and dodge.
Die wildling scum!
Robb gets Jon with a snowball to the chest.
Oh, I am struck! I am fallen!
Jon does a really over-the-top death scene while Robb waits impatiently for him to finish. Jon staggers about, then finally collapses dramatically in the snow just outside the front wall of the snow fort. Robb looks down at him.
Now get up and do it again.
Come on, Jon.
I can't hear you. I'm dead.
Robb sighs gustily, grabs a shovel, and pours a load of loose snow over Jon's face. Jon yelps and comes up off the ground with a jerk, spitting snow.
Robb! You're not allowed to do that!
I'm Lord of Winterfell. I can do what I want.
(shaking snow out of his hair)
You're not allowed to kill me after I'm already dead.
CATELYN STARK comes out into the courtyard. She looks at Robb in his snow castle and smiles fondly.
Go back and do it again, Jon.
No, it's my turn now.
Your turn for what?
I'm Lord of Winterfell. You be the wildling king. Come on.
You can't be Lord of Winterfell, you're a bastard. My mother says you can't ever be Lord of Winterfell.
Jon stops and stares at him. Robb's grin fades a bit when he sees Jon's expression.
At the edge of the courtyard, Catelyn narrows her eyes. She takes one step toward the boys, watching them closely.
There's a tense silence, then:
I can and I will.
He tackles Robb onto his back, knocking down part of the snow fort in the process. They tussle in the snow.
Jon, stop it! You're ruining the castle.
Say it! Say I'm Lord of Winterfell.
Catelyn appears on the scene.
Boys! Stop this, stop your fighting.
She grabs one boy in each hand by their coats and pulls them apart, still swinging at each other. Then she lets Robb go and grabs the collar of Jon's coat in both hands.
What did you say? Just now.
Jon stares up at her, scared.
She gives his coat a jerk, causing him to gasp in fright.
What. Did. You. Say!
Robb gives Jon a worried look and tugs at his mother's arm.
Mother, stop it. We were just playing.
Catelyn ignores Robb. Jon flinches with each sentence as Catelyn lays into him.
Never say that again. Do you hear me? Never say it again. Never think it again. Or I'll throw you out for the wolves.
Jon wrenches free of her grasp and starts backing away. His lip is trembling, and his breath is coming in small frightened gasps.
Jon, come back. I'll let you be Lord of Winterfell, I promise.
I don't want to play anymore.
He turns and runs. Catelyn watches him go and lets out her breath in a big sigh. Robb flops down to sit in the snow, legs splayed, staring morosely in the direction Jon had left. He looks up at his mother with a resentful stare.
Why did you do that?
Catelyn stares down at her son, conflicted.