Ok well, Ill get the ball rolling with whatever information I can provide, but ofcourse feel free to throw some of your own mind money into the kitty of knowledge.
My experience came from me living with siblings (in the same room as my three older brothers) so I've had alot of practice hiding stuff. lets start with the little things, that are easy to hide, for example, dummies (pacifiers)
I hid my dummies in a few different places over the course of my room and its many rearrangings, the first place I hid them was in a hollowed out dictionary. See my brothers didn't do alot of reading, neither did I at that stage but all our books were kept on a solitary bookshelf, and never touched. the thickest two books were a dictionary, and my copy of 'The complete short stories by edgar allan poe', considering I actually READ that book, I didnt want to cut it up, and Id already read the dictionary (I was a bored child) so I decided to hollow it out. I easily fit two dummies in there, and could probably have fit more.
If you dont have a bookshelf, or a hollow book that can be considered inconspicuous, the best thing (which is where I moved my dummies to when the bookshelf was removed) is to have a box or 'special things' that would be totally un interesting to the others, out letters and post cards and bits of old stones in it or twigs or something, and make sure you get it out filled with that stuff enough times for your siblings to see there is nothing interesting in it, its best to get a box deep enough for a fake bottom to be put in, but that would be an extreme I think. after theyve gotten used to it, put it somewhere out of the way and leave it alone for a while, then you can use it to store your dummies and other trinkets, and they'd hopefully not be interested enough to search through it (or atleast, mine wern't.)
Next ofcourse, is nappies (diapers). now these are alot trickier becuase your usually hiding more than one, and they are bigger, much bigger, than a simple dummy. I had a few inventive places to hide them, but to be honest my brothers were so wrapped up in thier own stuff they didn't really bother me much. The first place was an old camping bag that was in "Storage" whenever we wern't camping, by storage I mean on a high shelf in the bedroom, out of the way. the nappies were stored at the very botton beneathe some old shorts. This was good because as there were four of us, we each had a large camping bag (being avid campers,) and there was no reason for them to look in mine. The only problem with that was I had to move them whenever we went camping, and Id have to do it long before we started packing so that I could pack infront of them freely, ofcourse I packed my own bags so I didnt even need to mvoe em (I just didnt wanna chance taking nappies on a camping trip with me, although in hindsight it probably would have been fun!) another great space was in a large cupboard that was high up above the wardrobe, this normally stored bedding and (as my brothers and I rarely changed our sheets) was rarely opened. this may not work for you if you DO change your sheets, but one time I DID hide the nappies behind a pile of book on a shelf that was too high to see the back of, they were well hidden behind stacks of books and old games.
And the last place I hid them was a large bedding chest filled with my mums materials (she liked to dabble in textiles), that was risky, sespecially one time when she was showing a friend of hers some of the fabrics shes collected and I had to dash to the chest before they got there, luckily I kept my nappies in a carrier bag, so hopefully they didnt see anything!
But I cannot stress the importance of cleaning my OWN room, I suppose if your an only child your mum or dad would be more inclined to enter your room to get washing or do a bit of sorting out, but if I were you and serious about hiding nappies and not getting caught, Id think about telling your parents you want to start cleaning your own room, for a bit of indipendance or something.
I've heard of nappies being stored in floor boards, behind the fake walls of cuboards, inside matresses, in a loose vent shaft, boy the list is endless, alot of the time though, something will go wrong, no matter how good your hiding place is, so Id get some emergancy procedures thought up, either alternative places to hide your stash, or a very good excuse.
PS: I apologise for spelling errors, leptop keyboards still screw me up!