Vocal registers

Although the vocal registers are almost never mentioned in connection with children's voices, from my own experience I know that a problem with vocal registers commonly occurs also with children.
If you have the feeling that a child is singing with up to three "different" voices, or his/her voice skips or breaks on some tones, s/he has a problem with balancing vocal registers.
A vocal register is defined as a row of acoustically similar tones, concerning their timbre and sound character. When singing up or down, this tone row (tone sequence) changes individually in some vocal range frequencies into another row of tones which have a completely different sound character. The frequencies where this change occurs are called register transitions (register or voice break) and the tones are called transitional tones. (Only seldom do you see singers with a uniform voice without registers - they are called natural singers. Their entire voice range can be considered a mixed register. It is true that these voices usually do not have a range wider than two octaves, however, this is usually enough).
To put it simply, vocal cords work differently in different registers.

In singing terminology we usually speak about three registers:
Chest register
Middle register
Head register (falsetto)

The chest register is typical for the deep and lower vocal position.
The middle register is typical for the middle and higher vocal position.
The head register is typical for the high vocal position.

It may be interesting to mention that some authors speak only about the chest and head registers and claim that the entire middle register is comprised only of amphoteric tones (tones that can be sung both by the mechanism of the chest and head registers) when a combination of both vocal cord movements occurs. I personally think that it doesn't matter how we name the problem, but rather how we manage to solve it.

In general, when singing, men's vocal cords work mainly in the chest register and women's vocal cords in the head register.
The children naturally use the register that is determined by the structure of their vocal organ and their age. Younger children (and those with an anatomic structure of a higher voice in the future) mostly use a head singing register, because their vocal cords are shorter, and older children (and those with the anatomic structure of a lower voice in the future), when not trained properly, tend to mainly or only use the chest register. A typical example are the boy "natural singers" of folk songs, who may have a "big voice", but who have a very limited vocal range for high tones, because they almost always only use the chest register. When they try sing higher notes, their voice cracks or changes sound character, transitioning into something completely weak and voiceless D72. The reason for this is the inability of the vocal cords to fluently transition into the head register. The little singer works in the chest register for as long as it is possible for his vocal cords; often until they are completely overstrained. The moment when he has gone past the amphoteric tone range, his last chance to start mixing both registers, it becomes physiologically impossible for his vocal cords to create such high tones. Without these transitional tones, the vocal cords have to jump straight into the head register to avoid cramping. This type of singing is very damaging to the vocal cords. On the other hand, children who sing purely or (predominantly) in the head register, have an unusually small range downward. Their vocal cords do not "switch" into the chest register mode at all and therefore work in an unnatural way. This type of singing is typically very weak, whispery and quiet. Every attempt to make the voice stronger ends up either with the voice skipping, or does not work at all. D73.

For your information, in children the low transitional tone is around d1; the high transitional tone is around d2.