I am trying to imagine what would possess anyone to approach someone in that situation?
You are in a public place and you see a child that you think is Kyron. (Never mind that the kid looks nothing like Kyron and is obviously 2 years older.)
Under WHAT logic would you approach a child you believed had been abducted, while he was standing next to his abductor?
If it really WAS Kyron for all you know that abductor might take off with him and kill him. Or kill you AND him. Or at the very least would be tipped off to the fact that he had been recognized and take off.
I would take pictures...because I can do this without being obvious since these people wouldn't recognize me. I would take pictures of the vehicle and plate. I would hopefully be on the phone with LE already (or have someone else calling as I usually have someone else with me) but the pictures would be my priority to document it. I would follow them if feasible. But I would not approach them. My job is to gather evidence, photographic evidence. Not my unreliable testimony but evidence that they can look at and determine for themselves.
IF the opportunity arose to offer to watch the child for a while, I would do that. Such as the adult needing to go into the doctor's office, or the store, or something like that. (I have kids the same age and multiple times have been asked by complete strangers "Can my kid play with yours for a few minutes while I run inside?") Then LE could come while the adult was inside and eliminate that confrontation.
However...in Kyron's case. I am 99.9% sure that *I* would be able to determine on the spot if it was Kyron. There is no doubt that I would be more certain than the law enforcement who showed up, just by looking at him visually. They would simply have access to the information to prove or disprove it... as well as the resources to send a picture to Oregon LE for Kyron's family to look at.
You stand back...get the plate number, make, model and color of the vehicle. In that order. Then the description of the people themselves.