I'm genuinely confused by previous comments. I found the kids' perfomances chilling and convincing -- this is a movie about their relationship and nothing else. The adults aren't important (most of the time their faces aren't even shown), their acting is then irrelevant. And the powerplay in that dynamic is thrilling thanks to a fan-fucking-tastic script.
It takes guts to end your movie with the joyous glorification of child murder.
Do not be mislead -- this is not a supernatural drama. It's a pretty much straighforward unraveling of a simple premise: A little mischievous girl convinces her friend she's a witch and starts bossing her around.