I am the Carlos Kleiber of children's choir directors in a church setting ... I get on their nerves about practice, decorum, how they sit, how they stand ... and then we go have the house on its feet (although I almost fell out laughing at that one video when Herr Kleiber nailed some member of the orchestra with a dirty look during the performance when a note was missed -- that is so me, for better because it works, and for worse because now I know what I look like to my choir members). I never speak to them of their musical mistakes. I just make plans to go fix said mistakes at the next time of preparation. The most important thing at this stage to teach them is how to practice well and communicate clearly, and give them room to be human and growing.