Not the worst of the crowded early ’80s slasher field, but an artless entry all the same, straying from the playbook in all the wrong ways. Bloody Birthday is so enamored of its premise (essentially The Bad Seed cubed) that it can’t take its eyes off its pint-sized villains, which means we know exactly what they’re up to at all times. Some of their murderous mischief is for revenge, and some is indiscriminate, but absolutely none of it is a surprise, because the camera follows them every step of the way. And for such a precocious trio of killers, their bag of tricks is disappointingly shallow. Still, if viewed with some good friends over a few beers, there is some dumb fun to be had here.