There are two points of law involved here, neither of which are good news for you.
Firstly, the provisions of the Rehabilitation of Offenders Act don't apply to work with children or vulnerable adults. For such types of employment, convictions are never regarded as spent.
Secondly, even if a conviction is spent, an employer is not obliged to disregard it he becomes aware of it. For example, someone with a spent conviction could apply for a job (not involving contact with children or vulnerable adults) and lawfully answer 'No' to any questions about convictions. However, if the employer happened to remember seeing the applicant's court case in the press (or if someone else tells him about it), the employer can lawfully reject the application on the basis of the information available to him.
http://www.lawontheweb.co.uk/rehabact.htm
Chris