Revised: After a magical species abducts their parents, to transform them into their evil emperors, their twins must find and rescue them in time.

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After an evil specie abducts their parents, the twins must rescue them before they are mutated & brainwashed as their dark emperors

Q. why did i merge ?teenager and her twin? into ?the twins??
A. because it's essentially the same, isn't it?
Q. why I kept it seperate before in ?a defiant teenager with her nerd twin?
A. because they made an unusual team (the bold and the brain) which can be a hook in kid's film

Two protagonists would work better in your film since young audience will get bored watching a single kid roaming alone in vast space...
That being said, one of them should be the main protagonist,
I used "a defiant teenager with her nerdy twin" before, to exploit events such as "when the nerd kid needs saving by his-2 minute elder-sister" (my elder sister saved my wimpy a** many times lol) and it could be a running joke "i am elder, listen to me dave!"
it's just one lousy version. Use those twins that offer the most interesting plot.
good luck with your project!

As an evil species begins to mutate and brainwash their parents to rule their unified dark empire, a teenager and her twin must rescue them from a surreal fantasy realm.

Ok, this is bright pink blinking neon-sign highlighting who the bad guys are. A bit overboard?

I should probably rename the dark elves and make them a new, vaguely related, species. In the Star Trek universe, Borg assimilation takes a few minutes, I believe. The aspects I'm ?borrowing? from that, is that an individual is genetically mutated to become member of a different species. Some memories may be kept, but in a detached fashion. Loyalty is to the new species. For all intents and purposes the original person is dead. (In ST, it turned out to be reversible, here is probably won't be, but I'm still early in the creation process)

In this story I'm giving the kids enough time to save them, before it's too late. The parents being captured to become rulers instead of slaves or such, is making it more of an adventure/fairytale instead of too dark and allows for fun shenanigans in the rescue. Making the kids twins (thanks variable! ) opens up some great extra options.