There is a very interesting theory that correlates to that scene and the title of episode/chapter 1 "To you, 2000 years from now"
The other thing i noticed is there are no visible walls. Usually the terrain is flat and even with trees you can see the wall.
That Mikasa is wearing her trainee uniform, so she should be a teenager rather than an adult. Unless, of course she were a time traveller who stole a trainee's uniform
I'm guessing it's just an early promotional shot?
Though the walls not being visible in the shot of kid Eren is a bit odd.... hmm
it looks like eren is just in a forest, thats probably why you cant see trees. The mikasa one i dont know.
At first I thought they were two different scenes but actually it does kind of look like child Eren and teen Mikasa are in the same scene there, based on the scenery. As mentioned above, the landscape probably obscures the view of the walls, but this could also be Eren's dream from chapter 1, so things wouldn't have to fit. I often suspect the reason Eren's actual dream was cut from the anime was because it was too spoilerific...