I think that the Cloudbabies were close as kids, Kya and Bumi looking out for Tenzin and being protective of him when their parents weren't around, they just don't remember. This is, in my mind, is the last time they had fun together, the last time Tenzin smiled openly to his siblings, the last time they truly were close, before all they remember about those happy times, is the green flashes.
Eight year old- Airbender protege- Tenzin was excited. He got a glider. A real Airbending glider. Made for him by his father. He couldn't wait to show Kya and Bumi. His father had told him not to, but he wasn't going to listen. His older brother and sister were his best friends, he shared everything with them. He waited for later that day, when his mother departed for Watertribe business and his father was at a council meeting, to show Kya and Bumi.
Kya, named after their Gran-Gran who died when their mother was young, was his older sister, and second sibling, who, like their mother, was a Waterbender. Bumi, the oldest of the three, was named after their father's friend, Earth King Bumi, and was a non-bender. Kya and Tenzin didn't like to use their bending around him, they didn't tell their mother or father about this.
Anyway, Tenzin went into the living room. It was way past their curfew, but his siblings were playing cards in the living room to pass the time until they were tired. They looked up to see their baby brother, who never broke curfew unless permission was given, standing in the doorway, grinning like a maniac.
"Tenzin, what's up?" Bumi asked, before groaning in frustration and throwing his cards on the table.
Kya, grinning widely, took the rock candy from her brother, before turning to Tenzin, "did you have a bad dream, Little Bison?"
"No, Wolf Tail, I didn't." Tenzin said, walking into the room. "I just really want to show you something."
"What is it, Little Bison?" Bumi asked, before Kya cut him off.
"Is it dangerous?"
Tenzin shook his head, "no." His grin got wider. "It's Mad Genius dangerous."
Bumi grinned widely, and Kya grinned hesitantly. Every time one of her brothers said "Mad Genius dangerous", it was time for dangerous fun.
"Well, what are we waiting for?" Bumi asked, standing up, a crooked grin on his face. Kya got up too, finishing off her rock candy.
"It's something dad told me to never tell you two about." Tenzin said, leading them into his room. The walls of the room were painted with the history of the Air nomads, their fathers people, while the carpet was a light blue, a symbol of not only the Air nomads but the their mother's people- the Watertribe. He had bookshelves lined with scrolls on Airbending stances and techniques, written by their father, as well as a wardrobe filled with Air nomad clothes. On the door to his room, there was a sign that said: Cloud Siblings Only.
"Well, what is it?" Bumi asked, looking around.
"It's this." Tenzin said, fishing his glider out from under his bed, and pulling out a few plushies and pillows he kept under there for his older siblings- something that they did as well.
"A glider?" Kya asked.
"You got a glider!? That is some serious Mad Genius right there!" Bumi said, geeking out. "Have you flown on it?"
Tenzin shook his head, "no, but I've practiced using dads."
"Why haven't you taken it for a spin?" Kya asked
"What fun would that be without my best friends?" Tenzin asked, walking to the door. He turned around, looking at them, before asking, "are you comin', or what?"
Kya and Bumi followed him out the door, smiles on their faces.
They stood on the roof of the house, the wind softly blowing around them. Tenzin tapped the glider, letting the wings extend from the stick, before getting into position. He leapt off of the roof, letting the wind carry him around. He smiled as he felt the wind tousle his black hair, before circling around for his siblings.
He turned the glider upside down, his siblings grabbing onto it, before turning it rightside up. The flight around was unsteady, they were wobbling a little, but Tenzin proved to be a gliding master, steadying them with just a slight flick of the wrist. They were whooping and laughing. Enjoying the breeze that ferried them along in the night sky. Then, As Tenzin took them down low to the ground, they were literally knocked out of the sky.
They crashed, Kya and Bumi into the bushes and Tenzin face first into the dirt. The glider, sliding across the ground on its own, was torn and snapped, now laying useless on the ground, in pieces and shreds, against a rock.
"You two okay?" Bumi asked, pushing himself up.
"Yeah." Tenzin remarked, not mentioning the bruise he felt forming on his side.
"I think so." Kya said, slowing getting up.
When the three of them were up, they grouped together.
"Sorry about your glider, Tenzin."
"It's okay."
"Dad's gonna be real mad about that."
"Says who?"
The three kids turned to see their father, Avatar Aang, on his sky bison, Appa. Their father did not look pleased.
"Dad.." Bumi began, moving in front of his siblings, "your home, early."
"What were you three doing?" Aang asked, his voice sharp.
"We were, gliding on air.." Kya said, finishing her sentence slowly when she noticed her father's glare.
Aang turned to Tenzin, a glare burning into his soul, before pointing to the house and saying, "get inside. Now."
Bumi picked up Tenzin before grabbing Kya by the hand and running for their lives. When they got inside, Aang sighed.
He looked at Appa, "as much as I hate to do this," he then turned to the broken glider, "I have no choice." He picked it up, using firebending to burn it to ash, before walking into the house too.
His kids were on the couch, Tenzin cowering against Kya and Bumi. "Who's idea was it?" Although, he knew. It was Tenzin's idea. If it wasn't Bumi's idea, it was Tenzin's, and very rarely was it Kya's.
"It was mine, dad." Tenzin said, getting off of the couch and walking towards him.
Aang sighed. He hated having to do this, but it was necessary if Tenzin was to become a proper Airbender. Being an Airbender meant detaching oneself from the world, leaving no loose ends to a physical existence, and so that was what needed to happen.
Aang moved fast. Putting a hand on Tenzin's forehead and chest, bending Tenzin's heart and soul, his memories and love for his siblings, and turning it into relentless teasing and pranks. When Tenzin hit the carpet, Aang knew it was done. He quickly did the same to Kya and Bumi, making them the tormentors of their closest ally. Turning them on him, making it so Tenzin didn't have anyone to turn to but him.
Would Aang feel regret later in life for turning two against one? For forcing friend to become foe? Yes, he would. All of this was going to come back to bite him in the blubber. Maybe not in this life, or the next. But, one day, it would all come back.