The first day my daughter attended Kindergarten I was a nervous wreck. After Kindergarten she was to take the day care bus. I was so nervous I took the day off and followed the bus from school to make sure all was well.
My situation went without a hitch but at the daycare another little girl got off another bus. She attended the daycare facility a year before but had since gone to another daycare. She recognized the name on the bus and got on. No one checked if the kids got on the correct bus. The little girl had moved and the daycare no longer had the contact info on the girls parents. It was a mess.
On my daughter's first day of kindergarten my husband, who had taken the day off, went to the bus stop after school to wait for her. Well, the bus showed up but she wasn't on it. He called the school who got in touch with the district transportation department. They radioed the other buses and found her -- she had gotten on the wrong bus. At the end of the bus's normal route they drove her home.
Stuff happens. The most important thing is to not panic. It's the parents' reactions in most of these kinds of situations that does the most harm to the child. In this story the kid was probably not traumatized until his mother went off the deep end. Even at the age of five, there are lessons to be learned about dealing with life's missteps.