Go to Site Info.
Select the Site Info tool from the Tool Menu of your site.
Click Manage Tools.
Check the box to Enable Lessons subpage navigation in the right tool menu.
Scroll down and click Continue.
You may now click on the top level Lessons page in the Tool Menu to display its subpages as links in the menu.
The subpages will display.
Note: Only one level of subpage will be added to the menu, regardless of how many additional pages are nested within those subpages.