How to undo pressing Don't Trust on Apple Watch
I got the same message on my Apple Watch and responded "Don't Trust" since I had no idea what computer was trying to connect. Then I read this article so opened the Console app on my Mac. Initially it didn't show my Apple Watch under Devices. I think I had to connect my iPhone to my computer first before both my iPhone and Apple Watch showed up, the latter with an exclamation icon due to the "Don't Trust" response.

I disconnected my iPhone from my computer and restarted the iPhone. I then reconnected it to my computer to check the status in Console again. I got the "Trust this computer" prompt on my Apple Watch again. After responding "Trust" this time and entering my PIN, the warning icon disappeared in Console after a few moments.

It should be noted that there may be delays before the Apple Watch shows up in Console. I think opening Xcode seems to hasten the discovery process but I'm not sure. Also, I have yet to see any Apple Watch log entries in Console.