Have to admit that CRBs don't hold that much with me TBH. Enough cases inthe media int he past to show that they are not that effecient - the only show up the people who have been caught and are out of date as soon as the piece of paper comes back.
I think you have to trust your judgeent and you know your child. I have to trust that once at school my DD will make her own friends and that she will be able to speak up, at least to her parents, if there are any problems. I know the parents from the school gates and you gradually know them a little more from coffee or the odd night out. Sometimes you only meet one parent, sometimes both.
The reality is that there are not child abusers around every corner or in every four houses your child visits. They are rare occurences, no matter what the like of the daily mail tells us.
However, if your child is too shy or clingy, then fair enough. Thy are not ready.
But some children are ready to have playdates and then sleepovers from 4 or 5 years old.