John P. Bloch

Create child themes from any non-child theme with the click of a button.
This plugin is multisite compatible; if used on a multisite network, controls for creating child themes will be in the network admin instead of the regular site admin. Pick the parent theme from the dropdown and name name your new child theme. You can also change the author name and select files you want to copy from the parent to the new theme.
Click the button and you’re all set!

Child Themify

Create child themes with the click of a button.

BeardedGinger

Designed to display the Parent, Child, and Grandchild hierarchy in the most straightforward way out of the box.
This plugin will provide a widget that locks the Parent page as the widget title.
If this is a child of the top parent, the widget will display the Parent as the title of the widget and a ul of links for all siblings of the current page.
If this is a grandchild of the top parent, the Widget will display the same as the scenario above with the addition of a sub ul of all siblings of this grandchild.
Test it out, if it doesn’t work like you want out of the box, this plugin isn’t for you.

Simple Page Hierarchy Widget

Displays the most logical arrangement of Parent, Child, & Grandchild pages.

Child Themify vs Simple Page Hierarchy Widget