Before employing, perform a criminal record check on your prospective worker. This is especially important when you are working with children and you may be assisted by the police in this case. Often there is a register where the names of any person involved in child abuse or harm is listed, to allow for easy identification of people who you might be interviewing. There are of course more simply and free ways to search records of the people you are interviewing, including simple screening searches on the Internet. You never know if the person who has applied is the person you will want to work with, but a quick Google search can go a long way.
If you would like to perform screening before employing, perform a criminal record check through the police. You can simply submit the names of the people who have applied for the job to the relevant department and they will search records to ensure that there are no prior offences of significance. Although a person may be free to work and not end up in jail, certain things may appear on their record including traffic violations, and other minor crimes. This will help you with the identification of problematic applicants, and will ensure that you do not employ a former criminal if you are not comfortable with that.
Remember that sometimes screening does not give an applicant the opportunity to explain what happened. In your application form give the person applying for your job the opportunity to list any previous criminal convictions, so that you can get the issue out in the open. You are obviously entitled to pursue screening or to approach the police to search records of people applying, and if you feel that that is necessary then before employing, perform a record check to see if there is any sign of a criminal history.