Imagine a spectrum of things listed in order from most to least likely to have a certain characteristic. In your example it is somewhat less intuitive, since there are only two things mentioned, rather than an entire list, so let's make up a similar example. Let's say we are talking about people hugging Dorothy. We might create a spectrum as follows:
You might think it most likely that the Scarecrow would hug Dorothy, and least likely that the wicked Witch would hug Dorothy. Thus, when expressing that everyone hugged Dorothy you want to indicate that there is some level of surprise in the fact that the Wicked Witch hugged Dorothy. Therefore instead of simply saying "everyone hugged Dorothy", you might say "everyone hugged Dorothy, even the Wicked Witch". In normal modern parlance I think that would be the most common way of expressing this. But there can be variations of the particular expression used.
Back to your example, the narrator is expressing that everything about them was china, but that it is especially surprising that their clothes were china. Thus, it's not just that they were all made of china, but that even their clothes were made of china. This could have been conveyed without using the word "to" at all, but the word "to" does create more of the visualization of traveling down the spectrum going from most likely to least likely. The fact (i.e. being made of china) could be applied up to any point on the spectrum; in this case it went all the way to the end of the spectrum and included their clothes.
I suspect, however, that most people writing or saying this nowadays would not use the particular construction of "even to". They would either use the construction mentioned above (without the word "to" at all), or they would add in another directional word, such as "even down to their clothes". And if they are already using the construction of "down to", it might be simpler to leave out the word "even", and just say "they were all made of china, down to their clothes".
Note, though, that as this is a work of fantasy (and a particular type of fantasy that deliberately creates outlandish things), you you might find that your own assumptions of most or least likely will not match up to the book's.