If you have a backup of your child theme prior to the changes, you can replace the bad copy on the server with the backup. (Download a copy of the bad one so you can compare the two versions and bring back any stable changes that you had previously made.) Delete the old one before uploading the good one.
If you can’t access your admin pages, you’ll need to use an FTP client (like FileZilla), or your host’s cpanel (or File Manager) and navigate to the files on the server.
In your WP installation there, find wp-content/themes/'childtheme' and rename the active child-theme’s folder by adding something like -hold, or delete it altogether. That will disable the theme and hopefully allow you to get back into WP admin. WP will use a default WP theme in its place (probably Twentyfifteen).
