I'm amazed, reading reviews here and elsewhere, that nobody seems to have listened to the plan the aliens outline to Karen Kingsley when she becomes Guardiana / Safety Woman.
It's difficult to make out what they're saying, but the gist of it is that the aliens are expanding their empire, and they need Earth's children as slave labor for new building. As such, the aliens want to make sure that the children don't kill themselves and reduce the available workforce.
It's a wonderfully goofy film, but everyone seems to overlook the sinister motive.