There are a number of points to be made against this argument. Let us look at the two most important.
First, we do not know that capital punishment actually deters people from committing murder. Therefore, it is not clear that executing juveniles will do anything other than allowing us to gain vengeance for the actions of the murderers.
Second, there is still the issue of moral culpability. We believe that juveniles below a certain age are not fully capable of considering their actions and fully understanding the consequences of those actions. We believe that this separates them from adults. If this is true, they cannot truly form the intent that is needed to make a murder into a capital murder. By this argument, it is unjust to execute people who were not mature enough to fully understand what they were doing when they committed a murder.