Child themes & updates?

First thing first, that’s a very good question! Now let me tell you what I’ve done to definitely kill the annoying proceedings you have described.

Keep WordPress under Git Control.

There are a multitude of articles on the web that can guide you from the basics to advanced steps. I used to repeat the proceedings you mentioned over and over, and let’s not forget to mention the FTP-ing tedious part.

I’m not a sysadmin person, however, it was pretty easy for me to achieve a safe and optimized “Git Controlled” environment using Github.

Learning the basics about Github is mandatory, however, when you get to it, you would be able to achieve an “Updates & Maintenance” scheme to do almost all sorts of automated tasks for you, including syncing between your development and production web servers.

I hope it helps, good luck!