All I can fins online is how to disable previews, but I want to enable them. I can only see the sender and message content on my main admin user, but I try to use it as little as possible.
All notifications on the lock screen are disabled, I don't know if that's relevant.
AFAIK there is no way to enable previews. I believe it was originally designed that way for security reasons. I ended up changing my whole multi-user profile setup after I realized this.