In German, wunder means wonder, and kind means child. So the German loanword wunderkind literally means wonder child.1 In English, though, the word usually denotes (1) a child prodigy, or (2) a talented person who achieves great success at a young age.2
When words enter English from other languages, we usually italicize them until they become well known. Many publications still italicize wunderkind, but the word has been in English for over a century and should be known to most English speakers, so the italics aren’t necessary.
The German plural of wunderkind is wunderkinder. But in English we usually pluralize words, even words from other languages, according to our own rules, so wunderkinds is the more common form (and it’s less confusing to readers who don’t speak German).
One more thing: In German, nouns are capitalized, so wunderkind is written Wunderkind. But, again, we do things differently in English, and wunderkind has been in English long enough to have earned treatment as an English word.
Mozart is, I believe, the true Wunderkind in the magical realm of music. He began to play at so infantile a period that no date is assigned. At four he could play minuets. [Popular Science (1886)]
Is he the wunderkind talent who emerged at 16 with a self-titled album and a hit single, “Run It!”? [Pittsburgh Tribune-Review]
The ICA has wunderkind twentysomething Jacob Kassay’s singed silver mirror paintings, which seemingly came from nowhere to reach shock auction prices last year. [Guardian]
Even wunderkind James O’Connor has come of age, turning 21 in July. [Australian]
There is also Reema Major, a 16-year-old wunderkind from Khartoum who speaks fluent Arabic in the Sudanese dialect. [Globe and Mail]
That’s why Gordon, a wunderkind of New York fashion at only 26, has plenty of black in his Spring 2013 collection. [Washington Post]