My scenario, in which I believe Kyron is dead:
Terri planned this scheme, and enlisted people to help her but lied to them what they helped her with. ..
She knew the school would be busy that day. She probably even knew about the guy cutting the soccer field grass too. She came up with the doctor's appointment and made confusion at the school to place the blame on them. She lured Kyron out of that school and into the truck where no one could see him or wonder what was going on. I think she gave him a treat that drugged him, and then she hid him under a blanket in the back seat so no one could see him in her truck.
Then, she went about getting her alibi by running through Starbucks at one location and talking to someone she knew from the gym, plus the other store. I think after that she killed him by choking him, or just simply smothering him. Either way, it wasn't messy, and she buried him at a pre-planned place. She covered this, she thought, by driving around on backroads after burying Kyron just so she could say that she was driving around for the achy baby. She thought the school and some local perp would get the blame, and she'd be the sympathetic stepmother and wouldn't even be looked at.
Only she knows where Kyron is, and is probably not going to tell. Hopefully, those she lied to are starting to put the pieces together and realize what really happened. If they are still believing her, that means it'll be even longer until Kyron is found...