The primary concerns among police, prosecutors and judges is why the defendant is interested in child pornography (assuming they are guilty of course, since most police and prosecutors do, along with a few judges). The purpose of the evaluation is to determine if the client is actually interested in children. Do they pose a risk to anyone?
If the client already confessed and you are playing for mitigation or just challenging the case from a different angle, then there are some great experts that can fully evaluate the client. However, this becomes more difficult of the person maintains their innocence since a lot of testing instruments only work if the client agrees that they are guilty. Nevertheless, there are a few experts that are willing to work with people who maintain innocence. Having proof for the court and the prosecutor that your client is not a risk to anyone may go a long way to prove that the client should receive harsh punishment. It may also help the client get out on an ISP application.