I've given this a lot of thought... I think its not so much a matter of nobody having seen them as much as it is nobody having recognized them.
I think it would have been easier for a non-descript (normal, not creepy looking) person to walk out with Kyron without anyone remembering that they had seen the event, assuming they were moving quickly and the witness was watching from a distance.
My reason is this.... My son, E, is about to start the 2nd grade. There are 5 boys in his class (its a teensy school) from across the playground, parking lot, lunchroom etc... 4 of them .. same relative size, ethnicity, basic coloring, etc ...look alike to me. I have had trouble picking out my own kid in such circumstances. Also the boys who are in the first and third grades look similar enough that I'd really be in trouble if I had identify them if they were all in a group. BUT... I know the parents.. if I saw Mom of B walking with a kid, I'll assume the kid was B... but if I see an adult that I don't know then I won't really notice the kid at all. I'll assume its a kid I don't know since I know most of the parents of the kids I do know. Does that make sense?
If I knew Terri and Kyron, and I saw Terry walking out with a kid, I'd be able to say that I saw Terri and Kyron leave, with a good level of certainty. If I saw a person I didn't know walking out with a kid, I wouldn't be able to necessarily say for sure that I saw Kyron and an unidentified person leaving, unless I had interacted with them enough to recognize that it was Kyron. But from a distance, not likely. I think a kid might have had a better chance of seeing Kyron leave with a stranger than an adult would, but I think the kids were primarily otherwise occupied, in a place where they didn't have good visibility of the parking lots.