Ah, just saw your comment above Jane, so here's my suggestions:
> WP themes site
- Category for child themes (with lead-in explanation for visitors)
- Child category view displays link to parent theme
- Child theme page shows parent theme (with potential to download both child and parent or just child?)
> Personal server/logged in
- Link in themes view on individual themes to visit WP theme site and view child themes available
> Extra data at the top of child theme style.css
- Would be useful to have something like 'Parent Theme Version' to ensure child theme compatibility with parent theme.
I guess one of the important things to consider is making sure users are clear on the fact that child themes rely on a parent theme.