We have child proofed a couple of doors in our house by simply reversing the privacy lock sets. In particular we have used this method on the door leading to our basement stairs and the utility room in the basement. This is a really simple method to child proof doors without having bulky door knob protectors.
As you can see in the photo I’ve taken a simple privacy lock set (like you’d use on a bathroom or bedroom) and reversed it. I’ve placed the push button lock in the stairwell to the basement. I really have no need to secure the basement side of the door because there are deadbolts lock sets on the basement exterior doors.
We just keep one of the safety lock pins sitting on top of the door casing out of the kids reach. The safety pin comes with the door knob sets and it allows you to release the lock. Once the kids are old enough I’ll probably reverse the lock set again so that the door secures from the living space side instead of the stairwell. We also used this method to secure the door from the basement into the utility room (furnace and electric).
So if you’ve got little ones you’d like to keep out of certain closets or rooms you can use a privacy lock to accomplish that.