Of course, I realize that one has to qualify the definition of "child bride": in a sense, the consort of King George I of the Hellenes was that. After all, the new queen (born Grand Duchess Olga Constantinovna of Russia) was only 16 years old at the time of the marriage. It has been said that she was still playing with dolls and teddy bears, and in fact, brought along a trunk full of them to her newly adopted land.
So, using this qualifier: when was the last European princess bride under the age of 18? How about under 16?
Young royal brides seem to have been quite common, up through the end of the 19th century: Empress Elisabeth of Austria, born Duchess in Bavaria, was also only 16 at the time of her marriage. Her daughter Gisela later on was also 16, when she married Prince Leopold of Bavaria.