I remember a mindee falling over and hitting her head on a slide, which came up in a huge lump straight away. Phoned mum, really panicked as felt so bad so know how you feel, and they said "Oh we wondered how you had got away scott free for so long as she is really clumsy!!" I felt awful, but she was fine, apart from a nice black eye .
I think you can tell when something has gone on that is more than an accident somehow, and your reaction will have told them that you look after the lo fine, just sometimes they do have little accidents - part of growing up!