MainWP Branding Extension allows you to customize your WordPress (child) sites. With the Branding extension you can easily Add/Change the Global and Admin Footer Content.
This options provides you the ability to add custom content to your child sites Footer (Backend). Enter your content and click the Save Settings button. The content will appear at the bottom of all pages in the Front End / Admin area of your child sites.
If you want to delete the custom content , just delete the content from the text edit box and update settings.