You can just rename the child folder alone. Assuming this is the
currently active theme, activate some other theme while you rename the
folder, then reactivate the child afterwards. Changing the folder name
doesn’t really change the theme’s name as it appears in the admin UI. To
rename the theme itself, edit the header information in its style.css
file. Avoid altering the text domain value there unless you also search
and replace all occurrences of it in any of the various translation
function calls like|__(‘translate me’, ‘my-text-domain’)|.

Ref: wordpress.org/support/topic/rename-child-theme-directory/