From a report by the German news agency Deutsche Presse-Agentur reproduced here :

The court also heard that blood matching the dead man's DNA was found in a screw recess in one of the soldiers' rifles.

Of course in order to justify a conviction against a soldier on the basis of this it would have been necessary to prove beyond a reasonable doubt that a particular soldier had wielded the rifle, and inflicted injuries that killed the boy. The judge reasonably enough decided that this was not possible.

But the fact that it is impossible to determine who murdered the boy does not mean that he was not murdered.