Hi Dan,
I have the same configuration at mysalon.nl. I have the plugin network activated. In the network dashboard you can set the blog to use as default. In my case it is number 1.
In the dashboard of mysalon.nl on the tab XML sitemaps I checked the first select to produce the sitemaps. You can see which sitemaps are produced by clicking on the button in the second line.
I made no changes to robots.txt.
In your google webmaster page, you will see your sitemaps, in my case:
/sitemap_index.xml
/tbmc/sitemap_index.xml
The first is the sitemap for mysalon.nl and the second one for mysalon.nl/tbmc