Well I've seen all of them (except Pacific Rim, yet), but I didn't see that particular twist coming. I wasn't trying particularly hard to guess the ending as I watched either, but the only thought that struck me as I watched was that maybe all the adults were going to die, and Mama would stay with both children. Glad that was wrong.
I got the sense that it wasn't going to end happily and that was only because I knew what the director's films are like. I'll agree it's not obvious, I think I've just come to expect these ending from him.
