http://www.silive.com/news/index.ssf/2012/05/staten_island_mom_recounts_ord.html
Matron and driver claim they checked the bus. At that age, the kid would be in a car seat. Kind of hard to miss, I'd think.
If you know you can get arrested and fired for NOT doing something and don't do it, I think you'll find it pretty hard to find anyone with much sympathy.