what is a good hiding (aka watschen)?

"Watchens" mean beatings so I'm not sure what you are asking here?

Funny, I am reading the book thief right now and it's driving me crazy how the author is using some of the German words incorrectly. A Watschen is NOT a good hiding, it is the bavarian word for Ohrfeige, which is a slap in the face. A good hiding would be a "Tracht Prügel" or to get your "Hintern versohlt"

Yes, the word translates as a "smack on the ear" or a "slap on the face." None-the-less, my very German grandmother would use it as a replacement for "spanking."