I realize this is an old debate/argument/whatever-you-want-to-call it, but as a developer working on a customers WordPress site at 6am after being up all night, I now need a child theme.
I can call him and ask for his FTP info which I PROMISE he won't know, or a simple button would be a really great feature. I'm about to code a few lines in his current theme to mkdir and touch the files.
The fact that you provide the ability to edit the files but put a big warning on it saying "Editing these files is discouraged" and a button that says "Proceed" indicates that you do have certain developer-centric features. Another button right there saying "Generate child theme" would be perfect. (PS, the Appearance "Editor" feature is a feature of WP and not Jetpack right? I always have Jetpack so sometimes I'm not sure where the line is drawn.)
The feature is very simple, should be a fairly low impact feature (from a QA/future dev perspective), and really wouldn't hurt anything if a user did call it on accident I would think. Perhaps some extra check to see if their current theme is a blank Child theme so it doesn't continue to make nested children would be nice, too.