(Unless you are lucky enough to not have to hide it).
I have some Underjams and two soothers hidden between some clothes in one of my drawers. I also have about half a pack of cheap baby nappies stuffed inside a lockable plastic box in a cupboard. The rest of those nappies, two bottles, and a pack of adult pull-ups are in a backback in my wardrobe.
I am fairly lucky in that I trust my parents not to go through my stuff now that I'm 18, but I do still worry, since I sometimes spend a week or more away from home. I am on the lookout for a bigger box that I can put a lock on, but no luck so far.