Why should explain to children? It's enough just to say that this is one kind of money. And teen sit on the Internet more than adults, so if they want to know, Google will be the first who they will ask.
Because bitcoin is something that even a layman can earn. I'm just about to graduate college and looking back, there's shit ton of things that the teachers taught us but we never get to actually use in real life. One famous example is algebra and trigonometric stuffs. Never in my life have I used even the simple (a-b) * (a+b) = a^2 - b^2, and I'm pretty sure that I'll never use them at all.
Those are just trivial stuffs. To actually have all those trigonometry stuffs make money for you, you'd have to acquire high level skill like engineering wherein precise measures are needed. Bitcoin, on the other hand, doesn't require those complicated stuffs. You can literally make bitcoins just from luck. You can invest randomly in trading and then be lucky that the price of that coin has increased.
Kids have to learn about bitcoins not because they are going to use it on their career but rather it's an alternative way of making money. Others use it as a Plan B in case their day job goes South but end up going all-in on bitcoins because it pays more and consumes less time.