This is a very difficult, but also a very important, question. I agree with the earlier posters that, in general, the older a young person is, the more "responsible" s/he should be considered. I would prefer allowing prosecutors the option of charging young people as adults if the circumstances seem to warrant such a charge in the eyes of the relevant legal authorities, such as the grand jury, the prosecutor, the judge, and, ultimately, the jury, and courts of appeal.
Devil is in the details with this one. Humans develop at different rates. It's not that an 8 year old has no conscience or any sense of right from wrong when he/she commits murder, but there is also no way their brain and the connection between action and consequence is fully developed as an adult's is. That is why judges have to take this on a case by case basis. That is problematic as different judges use different litmus tests to determine whether or not a child has adult judgement. So I favor 18 being the hard and fast dividing line between juvenile and adult justice systems.
A 10 year old child can certainly show intent, but their rationalization is not likely to be of a mature enough level to warrant adult punishment. I agree with the above post that a 15 year old's maturity level is not fully developed and should not always be punished as an adult. However, some 15 year olds are fully aware of their actions and the repercussions that may occur, and they should be judged on an individual basis.
What do you mean by "children?" If we are talking about someone who is 15, yes, they can have intent. A 10 year-old? Not so much. But we also have to ask if someone who is 15 is as responsible as an adult. Should they be punished as harshly. To me, they should not because a 15 year old surely has less of an ability to think clearly and should therefore not be held as responsible for their actions.