It's an opinion right? We won't all see eye to eye but just because I don't agree with your opinion doesn't make it "wrong" or mine "wrong" --- Maybe Apple will add this feature? Apple got to where it is by having things that just work, simplicity - adding a layer of complexity, an option to password protect albums, will add a layer of complexity and headache for some people who forget their passwords. That's terrible PR - "Apple lost pictures of my wife that just died and I can't get them back because I can't remember the password!" - etc. or whatever.
Maybe integrating it with face-id and touch-id will help - but I'm just giving an example of the complexity that this ads to the platform that can cause problems. I think I'd call the 6 year old wrong or TiggrToo wrong. Having options are the spice of life. I'd support Apple adding it via the touch-id or face-id option.
I agree with TiggrToo that Apple designed this to be a hidden album from the main album, not an encrypted, lockdown, album. Who knows, maybe it'll change someday.