I have a cornet pupil aged 7. Her front teeth are very wobbly and on their way to falling out. Does anyone have any advice on whether she should play at this time? When her teeth do fall out I'm not sure if playing the cornet will effect the way in which her new teeth grow... any thoughts please? thanks

I don't have direct experience of this, but my wife knows several brass teachers who will not take pupils until they have their permanent eight front teeth, presumably because of the danger of affecting their eruption adversely. If you decide you can continue, now might be the time to concentrate on a low pressure embouchure.

On the other hand, I was told by a brass teacher that the mouthpiece actually rests on the jaw/gums rather than the teeth and hence has no effect on tooth development.

Take your pick!

Any trumpet players with wonky teeth out there?

My front teeth take some of the load when I play the horn with a mouthpiece slightly larger than a cornet one. Was your informant specifically referring to trumpet, cornet or orchestral horn mouthpieces? I would expect trombone mouthpieces to rest on the gum of the upper jaw, but for the lower teeth to be somewhat involved, with a fairly light load.

Not a cornet but a French horn.
In France the French horn is considered to be one of the instruments (along with the violin) which very young children can and a lot of them start when they are six. They all continue playing when their teeth fall out and while the new teeth are coming through. A friend of mine whose son plays the French horn said that the sound was slightly odd until the front teeth had come through completely - apparently there was a sort of background whistle - but it didn't seem to bother her son and has not effected the way his teeth have grown.