For kids who love monsters and/or gore: "The Little Vampire" series by Angela Sommer-Bodenburg is great fun. It's about Anton, a young boy who happens to befriend a vampire boy his own age (well, centuries old, but he died when he was around the same age as him). Lots of intricacies commence, as his new best friend also need to tackle the hunger pangs and the constant temptation to bite him when smelling Anton's delicious blood... (!) But he shows Anton a whole new world, he's able to fly etc. and as he can only go out in the nighttime (he needs to sleep in his coffin in the family crypt in the daytime), they have some wonderful adventures as Anton sneaks out with him to explore...
Anton gets introduced to the vampire's sister Anna, who proves somewhat annoying as she died in puberty and she's constantly trying to get his attention by dressing up in fancy moth-eaten clothes and stinky ancient perfume (*actually I'm not sure the gender roles here are totally sound... but let's just say that the characters develop a little bit more as the series goes on and later Anna becomes a less stereotyped character and we see other sides of her personality, I even imagine that she and Anton ends up falling in love at some point in the later books, this might all just be me misremembering and/or wishful projections, though...)
A running theme is the need to prevent other humans from discovering that the little vampire is in fact a vampire, and preventing the vampire families from discovering that Anton is a human boy (he sometimes accompany the vampires to meet other vampires, and to do this he needs to pose as a vampire, which of course proves a bit difficult and they're constantly on the verge of being found out...) I also vaguely recall some sort of psychiatrist(?) figure who turns out to be some sort of Van Helsing style vampire hater, but perhaps my imagination is making this up-
Also, Anton grows older, while his vampire friends stay the age they were when they died... As he's about to outgrow them, they toy with the idea to bite him to make him a vampire too, so that he can stay with them forever...
It's a long series and the individual books are a bit hit and miss, and it's probably one of those books you'll either completely love or not get at all, but everyone I knew loved it and we had a lot of fun reading them growing up... The drawings are wonderful too:
(Similar theme, different book: 'Fungus the Bogeyman')