Children's book with a read and black cover - the main character was a girl and she had a nanny or knew this lady who had a necklace made up of milk teeth.

[A]

I'm trying to find a book that I read around 10 years ago, so I think it was published between 2005-2010, but I'm not too sure. The book was a childrens book and I'm convinced that the front cover was a read design with a black background. I don't really remember the actual plot, but what I do remember is that the main character was a girl, and she had a nanny or a carer or something who had a necklace made up of her children's milk teeth. And i think the main character was quite an explorer who found a cave and somebody living in it?? Sorry this isn't much to go off but any help will be much appreciated!!

[Ayshe]

Journey to the River Sea maybe?