My small bridesmaid's brother did a bit of impromptu hairdressing on her, only three days before my wedding! Luckily it was only a chunk out of one side of her hair, and we were able to clip a lock of hair over the patch with a pretty hair slide, so it wasn't noticeable.
It happened because her dad, who was supposedly supervising all the children whilst they were having haircuts at home, let the 6 year old lad and his 5 year old sister make off to the hall with the hairdresser's bag of equipment, and didn't stop to think for a second that there might be sharp, sharp scissors in the bag!!
I do think there are real issues here - as others have said, it is not right that the children were able to take the scissors off into a corner. You can't watch every one of a roomful of children, all of the time, but for that very reason, you should be careful about access to sharp things, so that they can't make off with those to play quietly in the corner.
I also agree with those who have said that the nursery should be paying for the haircut to sort this out, as this happened on their watch. But I do agree with those who have said it is not the end of the world, and her hair will grow back.