You activated a child theme and everything seems broken? Don’t worry, there are two easy steps, that you need to make and your site will look great again.

Widgets

Widgets get reshuffled (e.g. the page sidebar widgets may end up in the header sidebar area). The best thing to do, is to go to wp-admin dashboard -> Appearance -> Widgets and reorder the widgets into correct sidebars. Once you do that, your site will already look better.

Customizer settings

If you have already made any customizer changes in the parent theme, before you activated the child theme, then you can export the parent customizer settings and import them into the child theme. You can do that with the Customizer Export/Import plugin:

1. install the Customizer Export/Import plugin,
2. activate the parent theme,
3. go to Appearance -> Customize -> Export/Import and click on the Export button, which will download a .dat file on your computer
4. now, activate the child theme
5. go to Appearance -> Customize -> Export/Import, in the Import section select the file you just exported and click on the Import button,

That’s it, the customizer settings should now be imported.
Interested in why this happens?

WordPress treats child themes as standalone themes, so upon child theme activation all theme settings get reset. This means, that customizer settings, widgets and other theme related settings are reset to the default values. If you have already configured any settings in the parent theme, before you activated the child theme, they will be reset, but don’t worry; if you switch back to the parent theme, they are still there.