Girls need opinions on a situation!

So my sister who is 4 goes to nursery I know other countries might call this day care and she goes there for 6 hours a day and then I pick her up at 3 o’clock because it’s close to where I work. Anyway on Thursday I was working and I got a call from my friend saying that she had just found my sister wandering the streets alone , she was wearing her nursery uniform so she returned her there and she asked was she supposed to be picked up by me which she wasn’t. Anyway I was panicking and I left work early to go to the nursery to find out what had happened. The teacher told me that my sister must have followed the parents out of the main entrance and escaped but the main entrance closes at 9 and it was almost 11 when I got the call!! It turns out no one even noticed she was missing until she got returned and the nursery is next to a busy road so she must of been wandering around by her self probably scared,cold and hungry. I was furious and I feel like the school should be reported she could have been ran over but my family are telling me to leave it and the school asked us not to report it as it’s almost time for new kids to apply to school and they will loose a lot of business if this gets out they said it won’t happen again. What would you do in this situation?