The main principles of a successful training

1) Never try an exercise with children without testing it first by yourself, making sure you have understood it properly.
2) Even in solo singing you should never allow the child's voice to lose its child-like timbre. There is nothing as sad as teenagers whose training made them sing like an opera soubrette of an unrecognisable age. If you follow the path of natural voice formation, you should arrive at a supported, cultivated singing, which still maintains a child- (or a girl-) like timbre YT 4 .
3) The lesson duration must be appropriate to the age and mental development of the child. A solo lesson should last no more than 30 minutes for preschool and first-grade school children.
4) Training of very young children should focus primarily on fostering a positive attitude toward singing. The training of technical skills should take the form of a game, so that it does not discourage the children from further work.
5) Approximately from the age of seven, children are fully capable of perceiving and understanding singing sensations, including the appoggio notion. Still, understanding this does not necessarily mean that the children are able to do it. Even adults need a relatively long time to practice the skill of flexible support, therefore you will need even more patience with children. At the age of seven, children will most likely achieve a subtle connection between the cavities and the diaphragm, while the capability of a real appoggio will develop at an older age. You can be more demanding with children of this age group, but I would still recommend that you maintain the playful method of training.
6) Try to select songs that the children will enjoy and that will correspond to their technical development. A repertoire that is too difficult will discourage them, one that is too simple or infantile will bore them.
7) Do not practice with children during colds and illnesses; children's vocal systems are far more vulnerable than adult's.
8) You can continue training with a mere runny nose, but make sure the nasal cavity remains open. If the nasal mucosa is swollen, it virtually rules out a proper setting of the voice function (the cavities are closed off by the swelling, failing to resonate).
9) For children suffering from allergies manifested by a blocked nose, the above principle applies too.
10) One absurd advice at the end, which is necessary because of several court cases: All the exercises with recommended tools (especially those of different kinds of fruits) should be done only at one's own risk. Possible drowning while mumbling into a small glass of water is entirely beyond my responsibility as well. Should you have any doubts about the safety of these exercises, I recommend doing them without the tools.