Been working on this for a while. It was never actually determined in the series when a bender's skills become evident, so disregard this if I'm wrong. I just…like babies? :)
Water
While his mother stirred soup at the fire, Sokka crawled over to the small bundle of furs resting against the wall of the house. Tucked inside was the newest addition to the family, and it wasn't the wolf pup he had wanted for his birthday.
It was a small, strange-smelling thing his mother and father called a funny name that he couldn't wrap his tongue around: his baby sister.
He thought he'd rather have a wolf pup.
Not that the baby was bad or anything. But when it was still inside his mother's belly, the way people talked about it made the thing sound like fun. So far, all Sokka had seen it do was gurgle and sleep. At least if he'd had a brother, he could have someone to be in charge of. But all this sister was good for was being something the grown-ups could coo over.
He sighed, glancing over to his mother, still cooking. His father was out hunting for the week, and all the older boys had gone penguin sledding, leaving him with no one to play with. The baby was sound asleep.
Which gave Sokka an idea.
If the thing was boring when it slept, maybe it was fun when it was awake. So slowly and carefully, he poked his sister's cheek.
And unleashed the most awful sound he'd ever heard.
"Goodness! You have quite a set of lungs, Katara!" his mother laughed, picking up the bundle and trying to soothe it. Sokka sat hunched in a corner, mittens clamped over his ears.
"Make it stop!" he yelled, nearly unheard under the screeching. Instead, the baby seemed to cry even louder, tiny mouth stretched as wide it would go.
And then the fire sizzled out—as a large portion of the roof caved in on top of it.
His mother uncovered her head to examine the now visible sky above their heads. "How on Earth did—" The baby screamed louder still, and half of the soup sloshed out of the pot, as if pushed by an invisible wave.
Sokka uncurled to stare at his mother in confusion, before letting out a cry of his own.
"I didn't want a magic sister!"
Earth
"It's the baby," said Lao Bei Fong, walking hurriedly ahead of the doctor. "Something's wrong with the baby."
In the middle of the room, the nobleman's wife stood next to the elegant wooden cradle with the nursemaid, looking up to the doctor when he entered. The Lady Bei Fong bowed her head.
"Sir, I think my daughter is sick; you see, she won't stop kicking."
He sighed, thinking that for all their money, nobles were surprisingly naive. "It is perfectly normal for infants to be restless, my Lady, it's only because—"
"Doctor, this is no after-birth effect. She never stops kicking! Not in her sleep, or while she's feeding, and she throws fits when she's wrapped in blankets. Something must be wrong."
Careful to keep a polite look on his face, he leaned over the cradle. Indeed, the girl was lying on her back, legs thrashing in the air. Every so often she'd hit the side of the cradle and give a squawk sounding almost of frustration. He felt the pulse and checked her head for fever. Perfectly healthy.
"May I?" The Lord nodded and he gently lifted the girl out. She yelped when he touched her, feet beating against his arms. A sick infant certainly wouldn't be squirm with so much vigor. Her tiny limbs thumped against his chest, then stopped. One foot rested over his heart, and suddenly the baby took a breath, as if she was about to cry. She didn't, and then slowly tapped her foot on his chest…in time with his heartbeat.
The floor beneath the doctor's feet sunk. The baby chortled.
"I don't think I'm the right man for this case," he laughed, as the Bei Fongs looked on in astonishment. "I suggest you find a Master Earthbender."
Then for the first time, the doctor noticed the baby's eyes.
"How long have her irises been clouded like that, my Lord?"
"Since birth. The midwife said it would fade within a few days."
Slowly, he moved his finger in front of the girl's face, while she stared somewhere off to the right.
"No, I don't think your daughter's kicking is the problem…"
Fire
The days after the birth were spent in quiet trepidation. The servants spoke in whispers, the townspeople gossiped in the alleys, even the Firelord's own council bowed their heads in the halls behind meeting rooms, all saying the same: that the newborn Prince would die.
He'd been born much too soon, shaking with fever, squalling day and night. The doctors and healers attempted what they could, but all shook their heads in the end. He was just too weak. Prince Ozai slumped around the palace for days; he had failed to provide his nation a son.
Iroh stood over the cradle, watching his new nephew moan and gasp, little fingers curling in the air. Despite everyone's morbid predictions, the Princess Ursa had named the child anyway. Zuko. It will only be seen on a gravestone, he thought bitterly. He looked on as the infant began to whimper louder, scrunching his face in a look of pain. His brother had been ranting the other day, saying he should just put the thing out of its misery.
The candle next to the cradle flickered, then sputtered out, cloaking the room in a murky darkness; curtains had already been placed over the windows for mourning. Iroh reached over, flicking his hand to summon a flame to relight it.
The young Prince sneezed; it sparked to life.
Iroh paused, flame still in his hand. He could only remember losing control of his own fire when he had been a young child. And then not even with this small of a flame.
He dashed across the room, tearing down all the curtains he could reach. Sunlight burst blindingly through, and the child squinted and cried in shock, but quickly calmed.
The candle flame grew brighter as he fell asleep, basking in the dawn.
It was said Prince Iroh was the first person to smile in the Palace that week.
"Zuko, you were born lucky."
Air
Her eyes followed one nun as she hurried to shut the windows yet again, rain splattering her orange robe red. Someone wiped her forehead.
"C'mon now, dear, almost there, almost there now…"
She clenched her eyes shut, cry escaping between the teeth meshed into her lip. A harsh snap of pain, and then—
A brilliant beam of blue light.
At first she thought it was her own mind, but when the glare cleared she could see the nuns, wide-eyed in shock, holding a bloody, squirming, squealing baby.
Her baby.
"It's…it's…"
"The Avatar Spirit…reincarnated…"
"I can't believe…with my own eyes…"
"We must alert the other temples, but they've seen the signal I'm sure…"
"Have to wait until he's older, to be certain…"
As her mind raced with the information, the windows had crashed open again, but no one hurried to shut them. The storm wind blew the rain in, whipping her hair to stick against her face and making the nun's robes flutter around them. The roaring weather soon drowned out their words, but she could hear her baby wailing still, the wind picking up tremendously, attempting to outdo his cries.
"There! Can you feel it?"
"An airbender! Spirits, it has to be…"
Finally, finally, they remembered her, still panting on the bed, and the baby was placed in her arms.
My baby.
"These things have to be done. I'm sorry…"
"When he's a little over a year, he'll go to the Southern Air Temple, it's safer there…"
"The monks will raise him…"
"He won't know who he is until he's sixteen, he'll grow up like the other children…"
"The world is changing these days, we need the Avatar to keep balance. So he has to be unattached…"
"It's a boy."
She nodded along, as her son's howling wind tossed her tears and mixed them with the rain.