I like Richiev's suggestion.
I am one who includes fantasy elements in many of my stories, and like this logline, I often ignore or change pre-established traits of magical creatures as I see fit, so I don't think it's necessary that a vampire can?only be destroyed by the traditional methods, like Richiev mentioned. However, if a vampire, even a young one, can be killed by a stray bullet, then what's the point of it being a vampire? In a story of mine, a hailstorm of bullets might rip a vampire's body apart and effectively destroy it, ignoring the established 'rules', but an accidental bullet? That's hardly a supernatural, superhuman creature.
To that point, why does it need to be accidental? What if the two human characters find this strange vampiric child and decide to destroy it because of fear? Burn it, decapitate it, expose it to sunlight. I think intentional would work better.