I’ve run into an issue where I am unable to open/expand CUSTOM HTML, Text, Image widgets in the Widgets page. Initially I just noticed those but after looking at it some more, it seems the majority of widgets are having this issue. When I add new widgets, they also have this problem. I’ve tried deactivating plugins to see if there was a conflict going on but that did not fix the problem.
The problem is not there when I switch to twentysixteen.
I discovered that i can edit the widgets when I switch to the parent theme so I was able to find a workaround for now. It only happens when the child-theme is active.
Did you make any customization in the child theme? If it doesn’t happen with the parent theme then there is some code in the child theme breaks this functionality.