Children and vibrato

Vibrato is an acoustic phenomena, created by a periodical fluctuation of intensity and frequency (pitch) of the tone YT01. The change in the sound intensity ranges from 2 to 10 decibels, the change in frequency (5-7 periods per second) represents a quarter tone to semitone on average. These values can vary with each person depending on their voice position, volume, and other aspects of the situation. Vibrato usually disappears in tones that are sung piano (at low volume).
Vibrato is caused by two mechanisms of the vocal system: by the pendular movement of the entire laryngeal skeleton up and down, and by the springing movement of the diaphragm.
Vibrato appears by itself in the course of vocal training, depending on the advancement of coordination between the phonation muscles, and documents the technical sophistication of the voice.
During vocal training, vibrato appears first purely in the sound intensity (supported by a flexible breath). Later a moderate, gradually intensified frequency vibrato (vertical movement of the larynx) joins in. As a result you get a combined vibrato (tone intensity + pitch).
Numerous books on singing state that once you have adopted a vibrato, you can never get rid of it. From my own experience, I dare say that this is not true. Or rather - it is only half the truth. Singers with a poor vocal coaching and insufficient voice control will obviously not be able to cope with suppressing their vibrato.
A good singer, on the contrary, will have no difficulty consciously reducing the vibrato, or even singing in a completely straight voice, i.e. without a vibrato. It can be quite easily achieved by imagining the sound you want to produce. If you fully control your voice, your brain centre, under the influence of the image, will perform certain amendments to the voice formation which result in subdued or completely suppressed vibrato (firming the diaphragm, etc.).
In any case, it is advisable to always have the developing vibrato under control during the training, as certain individuals are more prone to vibrating than others (resulting from the anatomy of their vocal and respiratory systems).

The tremolo is a vocal defect and can hardly be confused with a vibrato. It is an unaesthetic, whiny, fluctuating and uneven tone, where the pitch fluctuation is clearly audible to the ear . Tremolo is a typical sign of bad vocal training, but may sometimes also appear in older singers after years of singing, or in those individuals with greater tendency to vibrate.
The tremolo is the opposite of a rigid, blank, forced straight tone.

Forced straight tone
A correctly led voice may have such a small amount of vibrato (either deliberately, or just developing during the training) that the perception is that of a straight tone that is pleasing to the ear .
A forced straight tone, on the contrary, is formed in connection with rigid abdominal and pectoral muscles, which then cause cramps in the larynx and vocal cords. The tone is an annoying sharp sound without vibrato . Such a forced straight tone can be eliminated by adding elasticity to the breathing muscles resulting in a relaxed, not forced breathing.

Coming back to vibrato, I dare emphasise that it can only be beneficial and acceptable for the singer if it is achieved spontaneously during the training.
Unfortunately, it is no exception that a teacher requires that students sing vibrato which they should "somehow" achieve themselves. I was even confronted with "singing schools" where a kind of artificial vibrato was taught; they even categorised the vibrato, giving it different names. Any kind of artificial vibrato however is erroneous and has nothing to do with a natural and aesthetic voice. Where improper vocal training hinders the development of natural vibrato, no artificial vibrato can help (no matter how scientifically categorised it is).

Children singing with vibrato - yes or no?
The answer is:
Yes, if it is a natural vibrato developed by long-term training.
No, if it is artificially formed.

Unfortunately, such an artificial vibrato is not exceptional in children. In most cases it is formed by mimicking the singing of a poor teacher or of an idol. It is therefore not a genuine vibrato, developed spontaneously, but an exact mimicking of the heard model, appropriate rather for a top imitator. (Can you imagine how beautifully the children would sing if they had a truly good singing model?)
It is then obvious when you hear a five-year-old girl singing with a vibrato (verging on tremolo) worthy of a retired opera singer, that it is definitely not an acoustic phenomenon developed spontaneously during long-term training.
A different situation would be a ten-year-old child who has had intense vocal training since the age of six.
If you spot a small, pleasing vibrato in such a voice, it is a completely natural feature relating to vocal development. In no way would I consider vibrato a measure of a vocal sophistication of children's singing, particularly due to the fact that some individuals are more prone to vibrato than others (as already mentioned earlier).

It is very important that the resulting vocal performance be cultivated and natural. Nature will decide for itself when and to what extent vibrato should appear in a well coached voice.