If they weren't transported that far, why bother to take the time to tie each limb then?
Also, if one killer had two boys -- one in each hand, say, by the bindings -- wouldn't the bodies make contact with the ground below? You even say yourself, that the slack in the bindings is too long to have prevented them from escaping the ditch....you would in turn have to assume, then, that the slack would have been too long to prevent their bodies from making at least some contact with the woods.
These boys were barely conscious, if that -- the bindings (even with the slack) would have been sufficient to prevent them from escaping the ditch; at least, in the killer's mind.
In all honesty, it would have been both easier and faster to simply throw two boys over each shoulder and run 100 yards, than it would have been to waste time tying each limb and carrying them in each hand by the bindings. The killer would have to be just as strong/big to perform either option -- why would he chose the more difficult one? It would have been harder for him to run, in that there are two lifeless bodies most likely making contact with your legs; and the majority of the weight would fall on each the killer's arms, which would need to be at least somewhat flexed to hold the boys and keep them from touching the ground (unless the killer is eight feet tall and can simply let both arms with 2 bodies hang down, without either body touching the ground). If it's like you say, and the majority of the bleeding was internal, he wouldn't have to worry about blood on his clothes.