I think the easiness or difficulty is pretty evenly matched, IMHO.
For Terri, if she's guilty, she had the benefit of planning, she had the benefit of knowing the school's policies, vulnerabilities, and layout, and of course, she had the benefit of Kyron's trust. What makes it harder are many of the same things, IMHO. If she knew the school, the school and its workers knew her, and it is likely many other parents and students knew her from her time volunteering in Kyron's classroom. She is not someone who passes under the radar, in terms of her appearance. IMHO, she's not particularly striking, but her hair stands out. It's not just red, it's very, very long. The baby is very unusual looking and very cute. Anytime someone has a baby, people seem drawn to them. "Oh, look at the baby! Oh, she's so cute!" And on and on. The logistics of staging an abduction while dealing with a toddler are difficult to imagine. I think Kyron leaving with Terri out the front door would be noticeable; it would go against the grain, someone could easily ask why they were leaving, if he was sick, etc. In retrospect, no one saw her, but the risk for her was there even moreso than a stranger, IMHO.
As far as a stranger goes; if the stranger is not someone who came to the school on the invitation of someone participating in the Science Fair, then the stranger shouldn't have been at the school. There's the risk that someone could've asked this person what they were doing there. However, the Science Fair was advertised on the board outside the school, so the stranger would've known the school would be busy. He could have passed as a relative, especially if he kept a low profile. He benefits slightly here because no one is noticing him like they would have noticed Terri, "Kyron's mom," the baby, etc. The question remains, how did this person get Kyron out of the school unseen?
I'm pretty creative, and I honestly can't figure out how it was done. I know Turf Guy was busy, but if you're about to abduct and either murder or hide a child, you don't want to risk your entire plan because of a Turf Guy. I think he probably served as a deterrent for anyone absconding with Kyron on the access road, and then there's the chain that would've delayed a quick getaway. Turf Guy was pretty observant, seeing the children come out the back door, etc. The probability that a vehicle pulled around that side of the school, managed to get Kyron into the vehicle, and then drove on the access road totally unseen by Turf Guy at any point, has to be rather astronomical, IMHO. We're talking someone won the lotto with luck on that one, IMHO.
Was anyone toting around a large bin or duffel bag that day (ostensibly to transport a project)? Is it possible this was an accident committed by someone else, and realizing Kyron was obviously dead, they covered it up?
While we're considering possibilities...What if Kyron was killed at the school and removed later but before the searches? Were cadaver dogs used inside the school?