Also, for my brave Saturday Slash volunteers I will gladly do follow-up slashes (each more kindly than the next) on your query if you post them on the Query Critique board over on AgentQuery Connect. You'll get advice from me, and also people who are smarter than me. If you do post on AQ, be sure to follow the guidelines and let me know you posted so that I can follow up!
When twelve year old Maia Quinn arrives at her aunt’s mansion in a remote village of Grunewald, she is none too excited. With her parents away overseas, she has resigned herself to a long and dull summer, or so she thinks. You need a better hook here. Your first sentence is weighed down in detail, and I don't have any sense of originality here. So an MG MC reluctantly goes somewhere boring for her summer vacation, but finds out there's more in store? Been there, done that - why is yours different?
The night she attends the Start of the Summer Party, a mysterious flying trapeze artist brands her with a glowing silver star and then disappears without a trace. Ah - now this seems interesting. Trapeze artists and a brand? Get this in the hook. Soon after, the wall drawings at the mansion start to come alive, warning her that Grunewald is in grave danger. This is better, there are definitely unique qualities here but I have no idea how they come together. What is a Start of the Summer Party? Is this a village thing? A circus, I assume, since there is a trapeze artist. What do you mean by "brands her?" Does she have a tattoo now? And what does this have to do with the drawings?
Now Maia is determined to learn the truth truth about what? and even having a giant green hound for a stalker won’t frighten her away. She discovers that one of her ancestors was an immortal from the Faie realm, and because of this she was born a Wanderer, someone who can travel between two worlds. The immortals reveal to Maia that their race is in peril from the warring Nature Spirits, who were forced into a struggle by the thoughtless actions of a human. As a Wanderer, it is within Maia’s power to stop their conflict and bring both their worlds from the brink of destruction. That is, if she can survive the mission. Better - there's more detail here so we can understand the plot here, but you need to tie all these ideas into what has come before. What about those moving portraits? Why exactly is Grunewald in danger?
You need to make your plot more clear and tie these elements together in order for this to pop. It's difficult because as the author you already know the answers, and so instinctively draw the lines in between elements that seem unrelated to a fresh eye. Get those lines drawn clearly and you'll be in a lot better shape.