I'll say it again. Show proof that the kid does in fact know something.
The prosecution or the defense do not have to
prove the witness knows anything at all! It does not work that way.
If the child's parent hires an attorney that seeks to quash the subpoena they will present a reason as to why, and the requesting attorney may have to argue why the witness is important. It is rare that a court will dismiss a witness under subpoena without some compelling cause. Not wanting to testify is NOT sufficiently compelling.
And unless the requesting attorney is an idiot, it is very likely that there is good cause to show that the witness knows something relevant. Subpoenas are not issued in a vacuum. No decent attorney calls a witness that he or she does not have a good idea as to what they can or will testify to.
If the witness makes a statement that runs counter to what he told officers or other people, he can find himself facing perjury charges ... or, facing charges for false reporting or obstruction for lying to the cops earlier.