It seems like the aliens are bent on occupation, rather than genocide. Against that backdrop, their actions make some sense. For example, many have wondered why the aliens only attack at night. But what's the first thing occupying forces usually do? Impose a curfew.
Another big question is why the aliens only attack large groups. Perhaps such groups represent the threat of a large-scale uprising, which the aliens want to avoid. Not because they couldn't suppress such a revolt, but because doing so would require killing too many people.
The aliens harvest the children. Any accompanying adults are killed under the assumption they're parents who might try to take back their kids. But the aliens can't kill too many adults because that would mean no more children. They're following sustainable hunting practices.
This may even explain why the aliens are collecting scrap metal. Despite the EMP, some radios will have survived or been repaired by now. There were suggestions of this in episode 3, when the Colonel mentioned pockets of resistance across the country had made contact.
At some point, the aliens will need to use another EMP to prevent the resistance from coordinating. And when they do, their own electronics will have to be shielded. Perhaps that's why the aliens want aluminum roofing.They're building the mother of all aluminum foil hats!