Assuming you mean the charge "wounding with intent to cause grievous bodily harm" the sentence would more normally be one of imprisonment, just as it would be on an adult. However it is very difficult to name an "average" sentence, since the facts of each case and any mitigation on behalf of the defendant would be considered by the judge. There are sentencing guidelines - the maximum sentence is one of life imprisonment but this is very rarely imposed. Some factors which affect sentence include: whether the defendant pleaded guilty at an early stage; whether the defendant was in a position of trust/guardianship over the child; the degree of pre-meditation or "intent"; whether there was any provocation...far too many variables to confidently state an average sentence I'm afraid.