If my parents knew, they certainly didn't encourage it. On the other hand, I've lately decided that they probably did know, and hence that they were at least tolerant to a degree. And that's just a matter of statistics, really: My parents kept all of my sister's and my cloth diapers after we'd potty-trained, and I was sneaking these into my bedroom almost nonstop from age six until I left for college. And in my teens, I frequently had bags of Pampers and other disposable baby diapers hidden in my room as well. Several times, the scent of Pampers was quite noticeable, and these were some of the times I worried most about being caught. As I consider all of this, it's almost impossible for me to believe that my parents, who were frequently in and out of my bedroom, sometimes cleaning it, etc.--didn't stumble upon my diaper stash. They probably did so multiple times, perhaps holding onto a steadily dwindling hope that it was a phase, and that I didn't have some kind of fetish. But they never said anything.
And, generally speaking, my parents were pretty hands-off. I've mentioned it before, but my sister and I were given quite a bit of freedom. Unless we were doing something physically dangerous or something that threatened our grades in school--my parents were both teachers, so school was a big deal in our house!--we were allowed to sort things out for ourselves. That's not to say they weren't extremely involved in every aspect of our growing up. They just weren't hovering.