When interviewed, the father of one of the boys said that the motorway was unfenced, and just yards from their back door. All the boys had to do was slide down a slope to get onto the road.
However, it was dark when the boys were hit. Now, up north, it gets dark a littel later than down here, and this means it must have been about 8pm or later when they were hit. I think this is far too late for children of this age to be out alone.
Having said this, I am deeply sorry for their parents, friends and relatives as this is a truly tragic event and, as Paulham said, you can take your eyes of them for a few minutes and they're gone.