If you teach beginners of any age, but most particularly youngsters, start with head register development. Make sure they have a head register, that it is strong and CLEAR and that the vowel sounds are true and undistorted. Make sure the posture is straight, the jaw is loose enough to move and open, the face is alive, and the body is POISED, not slack. Make sure they can inhale without a lot of extraneous movement, especially in the upper body.
If you do this for quite some time and are successful, bring in some chest register on the bottom notes, using speech as the bridge. Make sure the sound is firm, not pushed, loud, but not forced, and bright without distortion into the nose.
Across the middle range pitches (depending on the voice type) sing in both a head dominant and a chest dominant sound, but keep the chest register light and easy and the head register strong and firm. Do this on a variety of vowels and musical exercises. Vary the volume from quite soft to comfortably loud. Then, expand up and down in range. Add in some consonants.
Come back to head register frequently to make sure it stays strong.
If you not know what isolated registration sounds like or how it functions, learn. If you do not know how to mix registers (and it has to be you, not the student who creates the mix), or are not familiar with these concepts, become familiar. They will save you and your student a great deal of grief and time.
Choose music that is simple and easy until the student can sing music that is simple and easy, simply and easily. Then, choose songs that are slightly harder in terms of range and power. Choose songs that are lyric-appropriate for youngsters. Do not let them sing songs about broken relationships, the sands of time, or being depressed. Stay away from extremes.
If you have a natural child belter, still teach head register, but do it as a protection so that the chest sound doesn’t get too tight. Do not assume the child will be better off learning “Caro Mio Ben”, in fact, assume the reverse. If you don’t know that belting can be done comfortably, and can’t hear what is correct (and that is understandable if you were not, yourself, a child belter or have not worked with one), go find a colleague who is and can help you learn how to listen. If you mess around with the sound and take it away, you might kill the child’s love of singing and he or she might never sing again. BE CAREFUL!!!!
Forget about the diaphragm. Forget about resonance in the cheekbones, eyebrows, nasal cavities, forehead, front teeth, or hard palate and forget about “singing on the breath” (it is the only thing you can sing on unless you are dead).
If you do not understand this, find out why.