Okay, I've just read this series, which is up to book 8 at the moment and is going to be completed with book 9 in August this year. It started in 2007 and you could say it started off as a series of children's book with the first book. But then it got progressively darker with each book. But when one of your main character's a talking/walking skeleton who was murdered when he was alive and then came back through the force of his hatred and desire for revenge on his murderers (ala CROW), what do you expect?
So the story starts out relatively simple. Young girl discovers she's can do magic, goes on wacky adventures, partnered with an older man/skeleton who's a detective. The magic's probably a bit more fleshed out in this than it is in Harry Potter, where wizards could just do whatever they wanted cause 'plot' and couldn't do other things cause 'plot'. The setting is a bit more Dresden files, with a sort of central government running things and being quite strict on people who mess with non-magicians. While there isn't the hatred for dark magic there is in the Dresden files, there's still a fair bit of disdain.
The books are generally divided into trilogies, with the first dealing with the Elder Gods of the Universe (and we're talking serious Cthulhu-ish shit there), the second dealing with the anti-Messiah and the third and final one dealing with the big Doomsday prophecy that's been building up in the first 6 books. Like Dresden files, the books tend to have a span of 1 year-ish in between books. That means while the main character starts out the relatively young age of 12 in the first book, she quickly grows to be about 18-19 in the most recent one. Part of this growth is shown in how the books have become progressively darker and darker. It's not Game of Thrones, but it's definitely darker than the Harry Potter series.
One might even say it's darker than the Dresden files if only because so many popular characters end up dying. A lot of them in really unexpected ways. It's not like they have this great big stand off and then die heroically. Nope. Dead. Suddenly. One of the long-time characters even gets possessed by a dark spirit and taken over forever.
That's one of the things that's morbidly appealing about the series though. The fact that it doesn't try to pull punches with its characters. I think MORE good major characters have died in Skulduggery Pleasant than Dresden files. I'm not going to go too much into it cause I don't want to spoil things and since this is really just a general review for books 1-8.
Overall, I think this is a pretty decent series. I don't really like all the characters but I can live with them. I can't say it's going to be a classic but I will say I enjoyed it more than I did Harry Potter. And this is considering I read harry potter at a fairly young/impressionable age and I only just read this series. So this will probably appeal to young adults more than it will children. Mostly because children don't like seeing their favourite characters shanked in the back. Note, don't let your kid read this unless you want to comfort them while they're crying.