Here are 2012's middle grade sci fi/fantasy books published in the US starring kids of color (and please please please let me know of any I missed!). Do not worry if you are pressed for time. It is not a long list; it consistes of 13 traditionally published, and 4 small press/ independently published, books.
First up are the books where the kids are shown on the covers. I am being very generous with my definition of "shown." In many, the ethnicity of the non-white characters is obscured or outright occluded. Then come the books where the text or interior illustrations are descriptive, including one where you have to read the book before it in the series to know that the kids have an indigenous Brazilian mother. The title links go to my reviews if applicable, or to some other informative page if I haven't reviewed the book.
I've also included the breakdown by publisher at the end.
The Cover Books:
The Book of Wonders, Jasmine Richards (The girl on the right is from a fantasy Persian Gulf-esqe area, ala Shaherezade. You can tell by her clothes.) HarperCollins.
Bridge of Time, by Lewis Buzbee (Jean, on the right, is from a Chinese-American family, something important to the plot. The back of her head looks plausibly Chinese American. So does the back of the boy's head. He isn't.) Macmillan.
Claws, by Mike and Rachel Grinti (the hardcover shows the cat, but the cover shown is the paperack sold through Scholastic school book fairs, which shows the Vietnamese American heroine front and center). Chicken House/Scholastic.
The Diary of B. B. Bright, Possible Princess, by Alice Randall and Caroline Randall Williams (Speaks for itself. A beautiful girl shown with no obfuscation). Turner.
Geeks, Girls, and Secret Identities, by Mike Jung (that kid in the middle there, on top of the title--that's Vincent Wu, the hero, small but with all his face showing! The two white kids are sidekicks. The awesome girl isn't shown.) Scholastic.
Dragon of Seas, by Pierdomenico Baccalario (four main characters, one of whom is Chinese. I don't think the cover makes his ethnicity obvious, but neither does it make it dis-obvious, and the dragon is awful cool and multicultural looking) Random House.
The River of No Return, by J & P Voelkel (I do not think that anyone could guess that Lola, the one on the left with the ponytail, is Mayan). Egmont.
Look Ahead, Look Back (2012) by Annette Laing (One of the dark shapes is an African American boy). Confusion Press.
The Savage Fortress, by Sarwat Chadda. (The brother and sister shown on the cover are Anglo Indians. Less clear is the fact that the dinosaur is actually an Indian demon). Scholastic.
Shade and Sorceress, by Catherine Egan (as Sherry pointed out in her comment, the heroine as shown on the cover looks to be of African descent. I must go back to this one and look more closely to see how she is described!) Coteau Books.
Ship of Souls, Zetta Elliott (I think it's reasonably clear that this kid's hands aren't white, but it felt like a stretch to call this a book showing a kid of color. Are hands enough????). Amazon Encore.
The Stones of Ravenglass,
by Jenny Nimmo (a very rare thing in mg sff--the hero is African.
Nothing about the way he's shown contradicts this (he has both hand and
hair of non-whiteness)...but it would be very possible not to register
it either). Scholastic.Shade and Sorceress, by Catherine Egan (as Sherry pointed out in her comment, the heroine as shown on the cover looks to be of African descent. I must go back to this one and look more closely to see how she is described!) Coteau Books.
Ship of Souls, Zetta Elliott (I think it's reasonably clear that this kid's hands aren't white, but it felt like a stretch to call this a book showing a kid of color. Are hands enough????). Amazon Encore.
The Serpent's Shadow by Rick Riordan. (The
Starry River of the Sky, by Grace Lin (I don't think I need to say anything about this one--Chinese boy clearly shown as Chinese boy). Little, Brown.
The Interior Description Books:
Above World, by Jenn Reese (It had been a long time since I'd read this one, and I was glad to be reminded by the author that "Aluna, the main character of two PoV characters, has dark skin. Dash, another of the main characters, is also not white.") Candlewick.
Chitty Chitty Bang Bang Flies Again, by Frank Cotrell Boyce (The illustrations clearly show that one parent is dark-skinned, and one parent is light-skinned. Thank you, all involved). Candlewick.
The Drowned Vault: Ashtown Burials #2, N. D. Wilson (You would not know from either the cover, or from the text, that the mother of the two main characters in this book is an indigenous Brazilian, so that by extension that they, with their dark hair and skin, aren't purely European. You would know this from page 234 of book 1, which I quote in my review of it). Random House.
The Mark of Athena (Heroes of Olympus, Book 3), Rick Riordan (a smorgasbord of non-white characters. I'm a bit doubtful about including this, as the mainest of the main characters are white. And, unrelatedly, why are they all American?). Hyperion.
So. There you are. If you want to give a kid a just-published fantasy or sci fi book, whose hero or heroine isn't white, you can chose from 13 books from big publishers, 4 from independent/self-published. If you want the cover to clearly and unequivocally show that kid with no silhouetting or other ambiguity, you can pick from maybe 6, depending on how you call it. If you want one that clearly shows a Hispanic boy or girl, or an Asian girl, you are out of luck.
And of course, if you want choice (!), if you want to browse a selection of fantasy books to give a black girl, say, that all star brave and beautiful girls like herself, so that you can find one that you really love and which is just right for her, you are out of luck. I am glad my local Barnes and Nobel sells Diary of B.B. Bright, but it wasn't the book I wanted to buy for my own niece.