Given the extension SandboxLink is enabled
When I am shown my own personal toolbox
Then I am expecting a tool link to page describing "what is my subpages"

As of this writing there are no such link.

The sandbox link makes an invite to create subpages, and thus there should be a tool to show the user which subpages s/he has created. It should be fairly easy to include a link to Special:Prefixindex.