Hey Chris,
VCE subjects in physics definitely cover some astronomy stuff and give a good grounding in science thinking. At a decent school they'll be able to select astronomy or astrophysics as one of their optional topics to study during the course. If your child is into astronomy and science, then this would be a great start. I would recommend also picking up one of the math subjects too.
This page, and
this PDF will give you a good idea of the various topics.
If they're academically minded then continuing on through a uni degree in science/physics would be great. Swinburne University has some fantastic astronomy resources and lecturers (I'm a little biased as I currently study there myself!)
As for certificate level courses, I'm not sure of any which explicitly cover astronomy. There are some graduate and post graduate courses although your child will have to have completed their VCE before attempting them.
To keep them interested throughout VCE, you could consider joining your local astronomical society. I'm a member of MPAS on the Mornington Peninsula so if you want more info about that, feel free to ask.
Cheers,
Carl