Apologies if I'm covering old ground.

I'd like to set up blogs for my family members, but I would need to establish a way of creating entries so simple that even a 10-year-old could do it. I see how to switch between basic, advanced, and custom entries - but is there a way to modify it even further?

Well, it depends what you mean by simplified, but the templates in question would be tmpl/cms/bm_entry.tmpl (for the bookmarklet) and tmpl/cms/edit_entry.tmpl (the new/edit entry screen) if you wanted to do a little hacking. If you give some more details as to what it is you do want, someone may be able to offer some more specific advice.

Ah, that helps a lot. I think I can modify that.

Follow-up - is it possible to have a different bm-entry template for each blog running?

Not directly. But you could include javascript code in the common template that displays different things based on the blog id.