A parent needs to have common sense AND a solid understanding of child development AND a close enough relationship with each child in order to know how how and whether any "harsh realities" are "introduced to them". A parent also needs to have the good sense and maturity and understanding of how to have a strong family to know that there are some "harsh realities" that one can refuse to allow in the home.
Many parents either under-estimate or over-estimate a child's intellectual maturity while often, and sometimes also, having little understanding of how/when (and sometimes whether) emotional maturity develops in any one child.
One may say there are "harsh realities" but then there are "ugly realities". The harsh ones can be something like having a very sick family member/grandparent. The ugly ones are things like what's out there more and more today - drugs, crime, abuse, negligence, homelessness, much of the stuff that is "realities" but doesn't necessarily or always have to become part of a child's/family's world (at least while children are young enough that their world is still largely their home and school).
My general belief is that childhood and youth can be compared to a type of incubation or, say, how bird's eggs "work" - if the process of "incubation" is cut short then all is generally not entirely well for whoever/whatever was "incubating".
"Protecting" doesn't, though, mean keeping all unpleasantness hidden from the child's view/experience. It's up to the parent to kind of edit (and sometimes "clean up") some difficult matters and understand how much a child of any age may want/need to know (or prefer not to, at least at the time).
Children who have been adequately but properly protected without being kept completely in the dark (a bad thing) tend to arrive at adulthood strong and able to handle "harsh realities" and/or better handle/understand the "ugly ones".
Children need to be able to trust a parent to be honest (without being ignorant and cruel) if they're not to grow up feeling alone and/or being people who say they "don't trust anybody". (I think most of us have met some of them.) And, even if they don't say it they think it, and it shows.
Some can't think for themselves and only trust what they read in a book by experts. Some listen to everyone and anyone who is not their parent or someone similar to their parent.
BUT, harsh/ugly realities that are in the "wrong dose" at the wrong time are, I think, more damaging.