A more accurate thumbnail of “The Babadook” is “Repulsion” for moms. Like the protagonist of Roman Polanski’s psychological-horror classic, Amelia is just barely hanging on to normal life, and when stressed and isolated by the increasingly creepy circumstances, she starts to lose her grip. To be sure, Kent’s made a monster movie, and uses the elusive title entity to unleash all manner of skin-prickling unease and big date-grabber shocks (and without cheap jump scares, no less). But the reason “The Babadook” will haunt you has far more to do with the way Kent flakes away the loving calm almost all parents do their best to project and exploits the anxiety and desperation and, yes, anger that sometimes roil away underneath. Because what are parents afraid of? Something happening to their children, yes, but also whether or not they are, or can be, good parents. And what happens if they can’t be? And what if there’s something really wrong with the kid? “It gets easier,” older parents tell their younger counterparts with a smile. But what if it doesn’t?