ALERT! ALERT! Thread moving slow! I'll post my thoughts on the child not being real!
Shepard sees a child playing on a rooftop, then the Reapers come. The child has a short amount of time to leave the park on the roof. Later in the level, you can see the same place where the child was at the beginning of the game- except that there is not park there anymore.
Next, the kid would have to travel a very long distance to the near top of another massive building. The child can be seen sitting near the edge of that building, watching a Reaper. As you move closer, he runs through a locked door to get in the building, which is later destroyed by the Reapers. A massive explosion happens in the room where the child is hiding .
When you enter the building, you hold open a stuck door for Anderson. When Anderson goes through, Shepard goes to an air vent with the child in it. The conversation with the child is eerie ("You can't save me."), what child refuses help from a millatary officer? When Anderson aproaches, the child is gone. And a Reaper growl sounds.
At the Evac sight for the child, nobody except Shepard seems to notice him. The child struggles to climb into the shuttle, and nobody even attempts to help him. When the child climbs in, he looks up, directly at Shepard.
After all this, personally, I believe that the child was never real. The child was fake right from the start of the game. I think that the Reapers are using this child to haunt Shepard about his failure to save him. The child also appears in Shepard's dreams, these dreams include lots of Indoctrination codex clues (oily shadows for one), and to top it all off, the "Catalyst" appears as that very child.