I've looked at the 2 rows of kids in the 2D concept art and pasted below them crops from screenshots which I think are derived from them. Some of them I haven't been able to find any matches though. Anyone have any ideas? I realize the guys in row 2 are a bit of a stretch, going by clothing cues in some cases.
Regarding the ones on the right side of the 1st image (I labelled 1D and 1E) do you think that just might be concept art for Conor and Greg? I was focusing so much on looking at background characters who might resemble them that I forgot to take into account the main cast.
That might explain why they appear alongside more notable supporting characters like Cameron and Cameron's sister. 1B (green plaid boy) doesn't seem significant, on the other hand, at least not yet... unless that's actually Night Ninja.
That would just leave 2A and 2E in the 2nd image to figure out.