Keep an eye out for puffing cheeks, raised shoulders and good posture. Involve the children in peer assessment, they can see just as easy as you if someone is puffing their cheeks. Young children don t need a lecture on buzzing, intercostal and diaphragmatic muscles. They don t understand words like embouchure, posture, arching tongues or even production and tone. It s important that the teacher is vigilant of these things, but in my opinoin the teacher should look, listen and alter accordingly.
Many teachers like to tell their pupils everything they know about brass playing.
I think that this is in most cases totally unnecessary. Holding a cornet properly for ten minutes is very tiring for a 7 year old. Make sure to give them plenty of rest. Posture needs to be checked regularly, especially with an upright instrument. Young children grow fast and their postures need to be adjusted frequently.