Well, I was a bit bigger than 12 inches, but Fievel Goes West: An American Tale.

Possibly because it was the only VHS we had in the house...

Anything by Studio Ghibli. Especially Spirited Away
The Iron Giant
The Nightmare Before Christmas
The Mask
Who Framed Roger Rabbit?
The Secret of NIMH

The Star Wars and Indiana Jones movies had been my favorites for about two years at that age, and I had just been introduced to The Fellowship of the Ring, which blew my mind. The closest I've come to being completely transported to another world through film.

All the 80s Schwarzeneggar/Stallone stuff. Alien/Aliens/The Abyss/Terminator would play well at that point. The Matrix definitely.

Would also be a good age to get them into Clint Eastwood with the Man with No Name Trilogy and Where Eagles Dare (a personal favorite).

Might be a little young for The Thing, but you could definitely get them into some of John Carpenter's lighter flicks, with Big Trouble in Little China/They Live/Escape from New York/LA.

The first 6 Star Treks. Pretty much anything with Bill Murray in it.

When I was 12 it was 2006. I liked Star Wars of course, Back then I liked the prequels and the originals pretty much the same because I was fucking 12 of course, I saw Revenge of the Sith 3 times in the theater, but no one tell anyone about that okay.

My other big love at the time was King Kong (any of them) and pretty much any other movie with monsters in it.

Yes I'm a toddler.

I agree with everyone's fine suggestions thus far.
I'll add:
Adventures in Babysitting
Need I say it: Goonies
Bill & Ted, 1 & 2 ("Bogus Journey" is the more kid oriented of the pair.)

Army of Darkness for sure. When you're 13 that's pretty much the greatest movie ever made

We're all waiting for Owen to speak up.

Oh this has reminded me of a more general question. I'm sure there are good ways and bad ways to introduce kids to Star Trek. It's okay if they don't like it. I tried out Doctor Who, Mission: Impossible (1960s), and The Adventures of Tin Tin on them to middling results.

But for Trek, do I try out TOS on them? Just show them Wrath of Khan? If you didn't grow up watching TOS reruns like I did, but you like Trek, how did you discover it? Teague has counseled that TNG will break an unprepared mind.

It didn't exactly break mine, but Season 1 made me an instant addict (I was 8 or 9 at the time, so any show with good spaceship effects would probably have that effect on me). It was my first Trek incarnation and still remains my favorite.

TOS was an action-adventure show in its day, but it can feel too slow and dated to a kid of the 2010s. The Wrath Of Khan sounds like a good idea, it should be a pleasant start for anyone who liked the original Star Wars trilogy (the same era, similar VFX techniques etc.).

I started with TOS, watching recorded episodes on VHS, so I'm probably the wrong person to ask

If you do TOS, do things like "Balance of Terror," "Corbomite Maneuver," "The Man Trap" and/or "Charlie X" to introduce the original cast. I think I rewatched Balance of Terror many times over when I was about 12. If they show interest, then other episodes can be shown.

Movies is like bullet3 said, and certainly a good way to test the waters is Wrath of Khan. Not really sure how to introduce TNG, but anything after season 3 worked for me-I can't remember which one was my hook. Certainly not season 1

I started with the reboot, then went back and did the original series/films, but if I'd had the chance to start with Khan, in retrospect I'd have taken it.