I work with vulnerable adults and am responsible for recruiting staff and doing crb checks. Our company policy is that it is in my discretion (same as nursery managers) whether to employ someone with a history. I have employed staff who for instance have had a shoplifting conviction 20 years ago. It should be part of the whole interview process and you have to make judgements on their trustworthiness now.
However, imo, I would not employ somebody with a history of violence, no matter how long ago it was, even if it was a one off drunken incident.
Also, due to stringent data protection laws, you have to shred the crb document once you've read it, and if you decide not to recruit due to what was on it, you cannot write down this as a reason on your interview form. You would be in serious breach of the law if you then went on to discuss this with the other staff or parents!
Like somebody said previously, you have to be able to trust the manager's judgement, after all, if there was an incident, she would be held responsible for recruiting this individual, so I'm sure the decision wasn't taken lightly.