I teach (but not anything musical) and I've found over the years that if you need to point out limitations, the most helpful model is what you might call the critical sandwich - start off with something positive ('enthusiasm' is usually a safe bet, not least because your students must be enthusiastic to have volunteered their time and made the commitment), followed by constructively suggesting areas for improvement, followed (the other 'slice of 'bread' in the sandwich) by a second positive point. In the unfortunate event of two positives being difficult to locate, one can always re-phrase the first one. So sort of soft/hard/soft, though the 'hard' should be delivered in a supportive manner.