Thanks! I have a theory about this game, I don't want to say what for spoilers though. Too bad. DO NOT READ FOLLOWING IF YOU DON'T LIKE SPOILERS!
The boy and his sister died do to the structure of the tree house once it fell. Once the boy died, he awoke to LIMBO and faced his Fear (Spider), Loneliness (Tribal people) Depression (Rain and Water) And see the death trap his tree house really was (Urban Area). And the times he saw his sister, she was digging for answers. After the credits, you see two corpses in place of the children, This is because after they've discovered what they've been searching for, they really have no where else to go, and that is was what was left after the tree house fell.