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. Explain that a car runs on petrol, lights work with electricity, fires and cookers sometimes use gas. That s why they need to take big breaths. Teaching a twenty five year old student will pose different problems than teaching a seven year old. Teachers should constantly assess if their language is appropriate for the age group they are teaching.