One thing I learned when I took "people" classes for management is to always start constructive criticism with a compliment. You might suggest that it has taken you many years to learn what you know about quilting and in the beginning you made many mistakes. Then ask if they would mind if you were to pass on some of the mistakes you made and help them not to make the same mistakes. Also tell them that you are so proud of the quilts they have made and that they have done a great job for their first quilt.