Yeah but that's too easy

Magnolia

Climax

Maybe City of God. Can't remember.

Emily Blunt's first proper feature role was very interesting - My Summer of Love.

IIRC she plays someone who is subtly manipulative or duplicitous and does it really well, along with doing all of the straightforwardly dramatic stuff perfectly.

Corio was my first thought. It's rare to see a child actor be so natural on camera.

The last child actor I was equally impressed by was Quvenzhane Wallis in Beasts of the Southern Wild, although she seems to be heading for only a decent career rather than superstardom.

I found that I could see the acting, one of the least impressive performances in the series imo. Glad you enjoyed her though.

Which is in itself a good answer.

It seems like over half of people who saw it had roughly the same response, which is kind of amazing.