Well I understand that but think of it like this: if you're going against a race of evil machines that seeks to annihilate all life in the galaxy and fighting them, you are going to see some gruesome details of it that will trigger PTS for Shepard aside from one kid blown up. Every time Shepard sleeps the dreams are becoming worse and more vivid like on Spec Ops to show the protagonist's vulnerability in the story of every time he/she lost a loved one, civilians died horribly when they were torn up by Reaper forces, and forced to make hard choices that will cost innocent lives but for the greater good.
PTS is less bad dreams and more having waking memory flashbacks and becoming actively avoidant of what may trigger them, or else flashback to such a point that you at least partially believe you're back to the traumatic event. In a way, Thane is more PTSD than Shepard in ME3.
And Bioware knows this. PTSD asari at Huarta. The kid reaper horn dreams are not believable. Vivid dreams getting worse would also not be believable. Bioware can do it but it'd be artistic licence about how specifically Shepard is dealing with it, but we'd have to have considerations then that Shepard is special even in this regard.
If anything, Shepard is experiencing trauma while during the war that is cracking his normally more monotone/stoic/strong/however-you-see-it facade/mind. Something lesser in ME2 (different stress levels affecting scarring and/or the reveal of what Shepard really is) becomes something more overt in ME3. Not so much post trauma but more being in the thick of things, having just enough time spent out of combat that maybe a little wittle bit of post-trauma might show. It'd be post-Destroy that we could imagine Shepard having PTSD and either retiring or going into deliberately less intense combat environments.
I don't exactly love the kid forest dreams but if I saw more vivid stuff then I'd either be super sure of indoctrination (only particularly due to Arrival DLC writing) or I'd just have to laugh at the lack of realism. Someone in Shep's situation and character, yet not under indoctrination, is more likely to just have really bad sleep than have such hellish dreams as you describe. Shepard with PTSD should be increasingly tempted to not seek a violent route, even if it is (compared to ME1-ME2) definitely much more dangerous and harmful to not engage in violence. The idea of the child as Shepard can speak to Garrus about him is fine, at least in Paragon dialogue - the weird creepy repetitive dreams is already silly though, and IMO your idea would have been even moreso. At least if striving for realism, which isn't necessarily Bioware's goal, but I tend to think they try to hit that for their characterizations, just not plots.
EDIT: To be completely clear - nightmares are not inappropriate. And your thoughts on flashbacks aren't entirely inappropriate. But the kid we got was pretty silly, and flashback dreams while in the middle of the traumatic time (the war) also isn't the best. IMO your idea would apply better for a 'post' ME3 situation, an understanding that if Shepard survives the Crucible, his trauma from it isn't going to stop, even if he can take healthy approaches towards it. There's good fanfiction on that, I think. The PTSD asari would be too traumatized to be back in combat so she has to spend extended time out of it, and so exhibits PTSD. Shepard at most has stuff like Citadel DLC, but even then, there's the included 'war buddies on shore leave' element that keeps Shepard in the thick of things enough that all the things there is more like a temporary reprieve than a 'okay you're really supposed to fully relax now like its peace time'. And I don't think Shepard's supposed to break from constant nightmares. He's supposed to be the solid foundation, just at least a little shaky as we get through ME3 and finally near broken right at the end.
EDIT2: There's crazier ideas of mine (that I do NOT take seriously as BIG predictions) that include ME3 being some sort of virtual Reaper dream cycle or something, in which case some sort of emphasis on the kid may be meaningful for future games. But that's into crazy theory territory, not as I take ME3 by itself - especially with Bioware's words so far of Shepard's story as 'over'.