"Jack! Put me down!"

My legs pedalled in the air above the Burgess Woods, being held up by the armpits by a winter spirit who I was certain had gone insane.

"Come on! It's fun up here!" Jack was laughing in my ear, which did not help.

"No! Really not fun!"

"Be boring then." Sighed Jack, "Just make an ice bridge."

"I can't concentrate when I'm hanging this precariously!"

"That's the point." Jack was back in teacher mode. "Say if you fell out of the sky, you'll need to be able to catch yourself before you hit the ground."

I grumbled because he was making sense.

"Lighten up! If you can do it then I'll show you something really cool."

"Pun intended?"

"No pun. You'll find out what it is if you can form a platform up here."

I looked back down at the ground and it swam dizzily in front of me.

"I know it's high." Said Jack, guessing what I was thinking. "You can do it, just ignore how far down it is."

"Easier said than done." I muttered, then took a deep breath through my nose. My hands splayed out and a platform appeared below my feet, which Jack dropped me on to. It held as it floated in the sky.

"Good job!" Jack spun around until he was hanging upside down in front of me. "Now, time for the cool thing."

He flipped himself back up and grinned. "Race you to that house!"

"Hey!" I called as he shot off towards a house that he hadn't even pointed out.

I shook my head and held my hand out, shooting out ice to form a bridge to skate on. I skated side on as I followed Jack, who had settled on a roof near the centre of town, frost spreading from his figure across the roof.

"I win."

"Stuff you." I replied, stepping onto the roof and making the bridge disappear. I sat down on the tiles and looked at Jack.

"So, why are we in the centre of town, where we can be seen? It's only nine."

"Almost nine." Jack looked up into the sky and I followed his gaze, seeing a gold coloured cloud appear in the sky.

I recognised it. It was Sandman's cloud, from where he delivered dreams. I'd seen it several times.

"What's so 'cool' about this?" I asked and Jack held up a finger, his mouth moving as he silently counted down.

"5, 4, 3, 2, 1."

Right on cue golden streams of sand flowed from the cloud and dispersed into different directions, hitting and passing through windows.

I let an impressed whistle as Jack laughed. "Right on time Sandman!"

He stood up and ran for the nearest stream, running his hand through it. Golden dolphins appeared out of it and circled him, jumping and making clicking noises.

Jack laughed again and pointed at another stream. "Go on, it's fun."

"Are you sure Sandman doesn't mind this?" I asked, standing up and balancing on the sloped roof.

"Course! Go on."

I walked up towards the stream and jumped up to catch it. The sand sifted through my fingers like silk and a little stickily and it burst into the scene of a snowball fight.

I spun on my toes to watch the various figures running around. I ducked underneath a tiny snowball that went soaring over my shoulder.

I had to laugh at the scene. It was just one of those things, it was a scene full of joy.

"See what I mean?"

I looked back over at Jack, who had a massive grin on his face.

"Yeah." I called back. "This is great!"

I turned back to the scene and felt like I could watch it for hours. After what felt like too soon the images faded and Sandman's cloud began to move on.

Jack sat back down on the roof with his staff across his lap, legs swinging. I made my way down the roof and sat down beside him, still with a stupid grin on my face.

"You, I think you guys forget sometimes."

I looked at Jack, completely confused. "Forget what?"

"Our powers, or abilities. We don't use them to fight. We use them to provide dreams, wonder, hope, fun. You use them to fight, but that isn't why they exist in the first place."

"I never thought you'd get philosophical."

I looked down at my own feet, not as bothered by the height now. "You've got a point though. But we are an army. We keep the Descendants of Pitch in check while you do your jobs. We aren't you, maybe we got our powers to do just that, fight."

"Maybe, or maybe you were supposed to help us."

I shrugged. "No idea. The Heads might know, but I've gone over what I know several times, genetically it doesn't make sense. I'm not directly descended from you, and you gained your powers after you died, right?"

"Right."

"So, it makes no sense that I got ice powers."

"Yeah it doesn't make sense. Maybe it doesn't matter that it doesn't."

"Of course it matters. This is world grounded in science."

"Said the girl with ice powers talking to Jack Frost."

"...Touché."

"Anyway, if you find out it takes the fun out of the mystery."

"Most people in HOG won't see that way."

"Well, they're boring."

"Speaking of boring, I've got homework." I said, standing up.

"Seriously?"

"Seriously."

Jack's sullen face suddenly split into a grin. "You called homework boring. Normally you defend it by saying it's actually interesting."

"It can be, but this one isn't. It's due tomorrow though."

"Alright, the King of Snowball Fights lets you off, citizen."

"Oh, you are too kind your Highness." I mock bowed and formed an ice bridge to get me home.

Reckon this happens around the time Jackie's sixteen or so.
It also puts into question the Descendants a bit. In my mind, the Descendants started off as an accident, from when various refugees from the Golden Age made it to Earth. However, as more modern legends appear, it gets a fuzzier. I will openly admit that the opening genetics do not make sense, I'll admit that. I'm falling into my own criticism about 'using magic as a plothole filler' as I used to explain it as 'magic and science mixing together leads to unpredictable results' and I refuse to keep pulling that. There could be some residual magic around, from the likes of Jack's transformation, but for Toothiana's and North's Descendants? No such luck. I could also say that the Man in the Moon played a part in that, but I can't actually agree with that. I'm under the impression that MiM is not as powerful as some make him out to be. I see him as more like Nick Fury commanding the Avengers. He's got the intel, and may very well have a plan, and asks the more capable people to handle it.
And now I've strayed from the point, how do family related Descendants get their powers in the first place?
Honestly? Right now? I don't know.
If anyone else can come up with a theory, I'd love to hear it, until then, I'm going to be sitting on this for a bit longer. I'm happy to go with Jackie's view that Descendants were created for a purpose, but by whom? 'Tis a mystery, and one I intend to solve.

EDIT 26/11/15
Got an idea!
Okay, since Some Descendants have been around for as long as humans have been around (at least in some areas) it wouldn't surprise me that someone like Ombric would be aware of their existence. He may have thought that, in case some of them get out of control, it would make more sense for some of their own kind to step in, rather than immortals or other mortals (I personally consider Ombric as an immortal long before the events of GoC, come on, he's lived since the fall of Atlantis!) so may have designed a spell to extend this to other, similar beings families This might also have been somewhat of an experiment to see how the Descendants would function.
Now, one thing I didn't mention, while I don't think MiM has a great deal of actual power, many people think that he does, so in those universes MiM would have had something to do with it (as he seems to do everything in those interpretations). So, yes, that's my theory.

Now to the actual one shot. An example of Jack's teaching methods.
I imagine Jack's lessons would be more on control and pointers on techniques that Jackie was learning rather than actually teaching her moves (although I bet he couldn't resist teaching her a few). This is because their two styles are very different. Jack is very focused on his staff, using it as, well, a conduit, something to conduct his powers with. Jackie on the other hand leans more to the bending style of cyrokinesis, using her whole body to manipulate ice, snow and frost, although it is mainly focused through her arms and hands.
Since the two have such different styles, it would have been very difficult for Jack to teach his moves, and even harder for Jackie to learn, as she would have to re-learn everything from the ground up. So rather than try to teach her his way, he focuses on what Jackie needs to learn, such as control in stressful situations.

And, I think I've gone on long enough. Shapeshifter out.