"Matěj, your cheeks are red."
Translation:Matěji, máš červené tváře.
"Matěji, tvé tváře jsou červené" could be probably accepted, it sounds fine to me.
However, "máš červené tváře" is a better translation. "Tvé tváře jsou červené" is kind of a silly statement that something is doing something/looks somehow. When talking about body we want to be more personal and use "máš." Can't really think of a better explanation.