Look, this movie is a miss. Every rating metric tells the same story: critics hated it (37%), audiences were disappointed (42%), and even generous IMDb users only gave it a 5.8. The Letterboxd score of 2.4/5 is particularly damning from the cinephile crowd.
The premise had potential - a teen gets pulled into a supernatural circus world and becomes a vampire's assistant - but the execution is forgettable at best. The 2009 CGI hasn't aged well, the plot feels rushed and confusing (they tried to cram multiple books into one movie), and it lacks the charm or scares to work as either family-friendly fantasy or genuine horror.
If your kid is obsessed with the Darren Shan books, fine, let them watch it so they can form their own opinions. But don't expect this to become a favorite or even a movie they remember fondly. There are so many better vampire/fantasy options out there - from the Hotel Transylvania movies for younger kids to What We Do in the Shadows for older teens. This one's a skip unless you're really scraping the bottom of the streaming barrel.