TORCHWOOD!
Okay so, re: the whole we want 10%of your children thing...
I can't help but think of World War Z. For those of you who don't know, it's basically political commentary with zombies, and worth a read at least. The short version of the part that I made the connection with takes place in Russia: after the zombies appeared, the military issued a communications blackout, and after several weeks of no contact with the outside world (or information about the nature of the emergency) and increasingly harsh conditions, the regular, noncommissioned forced mutinied. The mutiny was put down, and it was decided that one out of every ten mutineers would be killed. By the other nine mutineers in their randomly selected groups. With rocks.
The idea was that the guilt, more than anything else, would bind them together, and put down any rebellious ideas. Who wants to be responsible for the deaths of their fellows? No one. So they shift the blame to command- it wasn't their fault, they were following orders. They don't have any will of their own- it was all the governments doing. And that surrendering of their will to the government eventually helped pave the way for Russia to become a dictatorship (again). After all, if they don't have to have any personal responsibility for killing their friends, why should they have any responsibilty for anything else?
I think the 456's plan here is something similar- less that they need the children for something obvious, but that they need the cooperation of Earth for something such as giving up 10% of their children, so that afterwards anything they demand will seem easy. Thus far, they've been really good at manipulating humans en mass- they chose a segment of the population everyone has a large investment in (even if you don't have kids of your own, they still represent the future of the human race) to communicate through. They were very obvious about where they were centering in on- the language, the time, ect. even before the pointing. They didn't just induce a world-wide state of panic, but they placed uncomfortable scrutiny on London. That shows a fairly decent grasp of how to manipulate humanity as a whole, so I don't think that this sort of strategem is above their understanding of the human race.
We'll see what really happens, though.