Okay, so I've worked in daycare and I've seen this type of thing happen before. Many times actually. And I've shared this here before and I'll share it again: I've been the teacher to leave a kid behind. I still feel guilty about it and this was 6 years ago. I'm not the type of person to leave a kid behind either, it truly was an accident and it never happened to me again. Teachers are human and we do make mistakes.

That said...their reaction is really the part that I would be concerned about. Maybe they were panicking but the way they handled this was unprofessional. They should have immediately told you what happened, apologized and reassured you that he was safe. Yes, it sucks to have to tell a parent you fucked up. I had to call the parent of the kid I left behind and tell her exactly what happened and apologize. She was very understanding once she knew her child was safe, forgave me and still trusted me with her child but it still was a horrible phone call to have to make. I was prepared for her to yell at me and I wouldn't have blamed her.

If this were my child, I would ask the teachers and director what they plan to do to make sure this doesn't happen again. The child in my care was left behind because we were rushing to a special, went straight through two sets of doors and counted the kids as they walked through the doors. My co-teacher counted them wrong since they were moving and went I looked in the hall before closing the door, I didn't see the child because she had hidden behind the door I was holding open. We made a new rule that we had to stop and count the kids before going through each set of doors after my incident and always check behind the doors too.

I know this isn't the same thing but I shared details to point out that a responsible center and teacher will learn from this and do better next time. If you feel they aren't taking this seriously, then I would be worried. I'm so sorry your son was so scared. I can't imagine how awful that must have been for him.