R' Chaim Brisker (Hil Geirushin 6:9) question the gemara's (Sanhedrin 68) hava amina that a chiddush of the parsha of ben sorer u'moreh is that he can be judged and killed even before bar mitzvah. Beis Din can only accept testimony in the presence of the defendant and plaintiff. Since a minor is not considered capable of defending himself, there is no defendant present. How then can the court hear testimony and punish this pre-bar mitzvah boy?
R' Chaim answers that the punishment of the ben sorer u'moreh is not a direct consequence of the testimony brought against him. The witnesses do not and cannot point to any action the ben sorer u'moreh has done which would warrant capital punishment, and hence he is not a defendant against any crime. The parsha of ben sorer u'moreh is unique in that the Torah commands that he is killed al shem sofo, in anticipation his becoming a criminal, not because of any criminal act which he has already done.
By way of analogy, a snake may be killed because it is an animal which is mu'ad to cause harm (Bava Kama 1:4) -- the snake by definition is considered a dangerous animal; there are no need for witnesses to testify that this particular snake is prone to bite or caused any harm. Here too, the witnesses who testify against the ben sorer u'moreh merely establish his identity as a person on the road to criminal behavior -- the death penalty which ensues is a product of that identification, that legal status, not a result of the particular act the witnesses may have described or any current guilt.
There is a simpler way to explain the gemara's hava amina of reading the parsha that answers R' Chaim's question (which for some reason he avoids)... stay tuned.