In a situation like this, You need to use good judgement.
Was the member of staff aware that you have received the CRB Check back with history?
Were you aware about the history prior CRB check? By the sounds of it you were not aware.
You state there are 3 items one being theft when he/she was a minor. This now probably is irrelevant. But not to be forgotten.
Were the other 2 items the same i.e. theft? And were they much recent. You have to weigh it up and see the relevance. For example he / she might have one for criminal damage see how recent it was a ask them about it. It might have been a lovers tiff and he damaged the car or house door in temper.
You also state that this person has worked with you before and is good. So you also have to judge the persons character in the present day.
I have employees that have history. Depending on that history they are only allowed on certain sites "alone" or not at all. (Or worse still not employment with us).
Also when questioned about history you have to be careful and try not to think the worst. They probably will be embarrassed about it and automatically think you will not trust them.
Some people do change, some people do get charged through no fault of their own (This I am aware of) sometimes the law (police do get it wrong).
As for making the client aware, unless its in your contract you do not need to make them aware. (That does depend on circumstances) i.e school cleaning so if its a child molester they need to be banned.
Best of luck
Dave