"How shall I know if she is under proper instruction?"
A good teacher will, first of all, not allow her to sing at the top of her voice, which all children seem to delight in doing. A good teacher will not develop on the extreme high or extreme low tones. A good teacher will even up the medium register, teach her how to use the breath on the tone, how to place the tone, overcoming all seeming change from chest to head, will give her perfect pronunciation and enunciation. This can be done at any age from eleven years, depending on the individual. A girl who has good ear, and who does not lead the singing in school at eleven and sing at entertainments, can begin at sixteen or seventeen and develop into a very fine singer.
"Should my son take up voice culture before his voice has changed?"
This case is just the same as with the girl, if he sings. In fact, I have found in my twenty years' experience as singer and teacher that the boy who studies voice culture before his voice changes has an easy road to travel after his voice has changed. Many boys' voices have not finished changing until they are eighteen or nineteen years of age. The boy who studied before his voice began changing understands the breath control, the placing of the tone, and the pronunciation and enunciation. These four fundamentals are absolutely necessary in order to sing well; and whether his voice, after the change, develops into tenor or bass, these fundamentals remain the same, and enable him to continue, instead of merely begin. The boy, who has studied, or is under a good instructor, will know when to stop singing. I have known many boys with promising voices, who have ruined them entirely by singing or trying to sing during the change. But they were not boys who were under instruction, or they would have known better. I do not claim that it is necessary to begin the study of voice culture as a child, as this is entirely a matter of the individual, but I do claim that you can count on one hand the singers who have reached distinction and whose voices have lasted any length of time, who started their singing lessons after they were out of their teens.
I have pupils who are making a good living as church soloists and on the concert stage, who commenced their study after they were twenty years old, but they are the exception, and not the rule.
I think a woman has the greatest success in teaching children. This may be partly due to her maternal instincts. Her illustrations and demonstrations are more simple than a man's. Her patience with children also fits her wonderfully well to teach the child.
You can accomplish nothing with the voice through fear. If the young boy or girl loves the work, looks forward to the lessons, they cannot fail in whatever they undertake.
To satisfy those who disagree with me in regard to the value of early study, I would ask them to read the lives of the great singers, and they will find that with very few exceptions they took up the study of voice culture before and during their early teens. Space forbids me to give a complete list. However, for the benefit of those who have no access to the biographies of the singers, I will select the names that I am sure you are familiar with, beginning at 1740, and down to the present time:
Malibran, one of the world's most famous singers, at the age of seven was studying Solfeggio with Panseron at Naples, Italy, and made her debut in grand opera in her fifteenth year.
Pesaroni made her grand opera debut at sixteen, and twenty-five years later we find her still one of the leading grand opera singers.