[blowfish]

Why not just empty the trash? In the worst case, your family members will learn not to use the trash as long-term storage.
It’s like asking how to do a partial flush of a toilet.

[zeebee]

I know there has been a lot of sarcasm posted as replies in this thread, but there definitely is a way to delete selected files from the trash on a Mac.

It is even simpler than the Windows way.

Just select the item in the trash and hit Cmd-Delete and that item disappears.