Security Dashboard is designed to be 1st place any site administrator will go to, in order to learn of any issues that they need to take care of.
It provides a real-time, in-depth analysis of your WordPress site to proactively identify threats to security and stability.
If Overview section displays parent-child themes as inactive and that they should be removed, please understand that:
We have code in there to account for parent-child themes and you shouldn't be seeing that. Your themes weren't setup as you thought.
We suggest you to double-check on this, and ask your site dev to make sure your parent-child theme setup is as you expect.
Note: If the parent-child theme is setup correctly, but it's a multisite setup, then there's likely nuances with multisite that affecting this. This feature isn't tested with this...