Hello There,
Thanks for writing in!
You can access your child theme files via Appearance > Editor. However we do not recommended that because if you mess in there you wont be able to access your dashboard anymore unless you correct the error via FTP. So we recommend you access your child theme files under wp-content/themes/x-child and edit it with a text editor application (e.g. notepad++, sublime, etc.)
Hope this shed some lights.
Cheers!