Iterative design methods involving children and educators are difficult to conduct, given both the ethical implications and time commitments understandably required. The qualitative design process presented here recruits introductory teacher training students, towards discovering useful design insights relevant to music education technologies "by proxy". Therefore, some of the barriers present in child-computer interaction research are avoided. As an example, the method is applied to the creation of a block-based music notation system, named Codetta. Building upon successful educational technologies that intersect both music and computer programming, Codetta seeks to enable child composition, whilst aiding generalist educator's confidence in teaching music.