Learn the music you want to learn; learn it any way you can. There is no wrong or right way. Not everyone will want to perform or choose a music-related career, but that doesn’t mean they shouldn’t learn the most effective methods and use the knowledge of music theory to elevate their abilities. Reading musical notation and playing by ear are both acceptable and challenging methods, but I recommend any budding musician learn how to do both to a degree. Something that has made me both a successful musician as well as teacher is my ability to be flexible with how I learn and teach it. In the same way, versatile musicians learn to read standard musical notation as well as play or sing by listening and imitating what they hear. Each student is unique, and the degree to which they practice both methods will vary according to each students needs and desired outcomes.
The road to meeting your musical goals is paved with discipline and practice. The key to success isn’t necessarily the amount of practice, but rather the consistency and, more importantly, the quality of that time. Ten minutes of focused practice per day for five days each week will yield better results than a two-hour marathon session once a week. This is ultimately the responsibility of the student, but the parent should act as a coach, cheering section and enforcer (when necessary) to help good practicing habits develop.
In a nutshell, it all comes down to this tried and true adage: you will get out of it what you put into it.