I translate a short excerpt from the most famous German method for chorus: Basiswissen Kirchenmusik 2 (Chor- und Ensembleleitung), that is Fundamentals of church music, volume 2 Conducting of choir and ensemble), which among other things is defined text ecumenical, because it is intended to churches of various denominations.
On p.78 we find interesting principles of the corrrect posture of the singer, and of course these recommendations apply to children because the physiological fundamentals are the same for children and adults.
The text reads thus:
"The posture. Singer intends conscious his whole body as a instrument. A good attitude stems from a balance of active muscles and relaxed. A key element of our body is the spine. Given that the vocal muscles and especially the respiratory muscles (diaphragm) are closely related to the spine, physical exercises in choir should always be aimed at straightening the spine. The result will then automatically correct the position of the head and a free and open chest. Good posture not only improves breathing and vocal duties, but also the attention and concentration of the singers. Experience has shown that a posture in tension upright and natural, standing or sitting, it makes the singer attentive and motivated, and every director want an attentive cantor."
Are also given some practical advice:
"Only non-rigid body, but also not without support, is the basis for a good song. Physical exercises are mainly for straightening the spine. Exercises for posture and breathing should never be considered in isolation from context of the emission of sound."
It is provided with this picture, the caption which describes "the position of the diaphragm":