Actually, just to clarify Inapickle's answer, the defence lawyer does not have to be told of the persons identity, only the judge must be satisfied that the witness is bona fide. This is what happens with paid police informers, their details are normally never released to the defence even if they provide good evidence. A statement is not taken from the informer but their 'handler' provides a document outlining the information that they can provide. This normally means that the informer is not a proper witness as such and therefore does not get called to trial.
If the informer does provide a statement and is called to court, they will be given a false name for court purposes ie "Dave" and their identify and possibly even real voice concealed.
If the prosecution were ever to 'secretly reveal' informers details to the defence then it would a) completely undermine the reason for having informers in the first place and b) put the informer at risk of harm (not all solicitors are honest!).
Finally, it's worth saying that if you do know anything that would assist the police in the investigation into the murder of this poor child, you should help all you can.