For a start, it could have been cut down to three hours without losing much important. What the 456 was using the children for was fairly imaginative, but doesn't make too much sense - there are chemicals in children they used as stimulants/drugs, but for some reason they can keep the children alive forever (so presumably use the same child forever) and also make sure that although the child is fully conscious, it feels no pain. But for all their technology, they can't isolate the chemical and create it synthetically. Does the existence of such a chemical only in children even make sense? If it does, I wouldn't know. I can see they're getting at the idea of pre-pubescence, but...meh.
As I said before, I've seen quite a bit of Torchwood, and I'm not much of a fan. None of the characters are especially likeable, least of all Captain Jack himself, whose main trait seems to be that he'd roger anything that moves. So all the focus on the main characters didn't do much for me, and was a lot of padding.
It seems to me that they started out with the ending they wanted, namely Jack sacrificing his grandson to save all the children in the world, and worked backwards. None of that made much sense. The 456 can control all the children in the world to the extent that they can use them to communicate, and with a little jiggery-pokery, the signal can be turned back to kill it, but some child has to die for it. Great.
That said, the performances were mostly good, I wouldn't fault them too much. Still, it's a lot of padding for not much story. All in all, I'd give it 1.5 or maybe 2 out of 5. I'd have got better and more challenging sci-fi watching 5 episodes of The Outer Limits.