It would be a neat twist if they were after children because it is easier to train them as slaves or they're tender and thus taste better to the aliens. But in that case they'd want women too so they could just breed more to eat.
Major fail for not showing the invasion. I understand it's cable and TNT, and funds for original programing is more limited. But how can you have a show about aliens invading Earth without showing the initial invasion?
And as others have noted, a snore fest to boot.
I'm completely speculating, but maybe the kids are harnassed, then put into the mechs? Have they said if they ever got a good look at the remains of a mech after it was destroyed? It'd at least explain why they have two legs and arms instead of six like the aliens.