Using advice from many people on this site, i thought a plan for buying diapers the first time. This is pretty much my plan, and most of these ideas i credit to other users here. So here's the idea:
Wait till your parents are for sure going to be gone for a little while, and then leave to the store. Wear a hooded jacket, useful to avoid awkward experiences with cashiers. Make a fake list and make sure the cashier sees you have it. Buying candy or something else along with the diapers is really smart. It would also be better to have a bike to get home fast. An unused backback will work great, but you would have to leave it outside of the store for a minute. When you are there, look for the oldest cashier first. Then go get the diapers. Get something else with it and pull the money out of your pocket, not a wallet so they dont know its for you. If they have paper bags, definitely get one for them. If you brought a backpack, then thats even better. And try not to leave too quickly.
So there you go. No advice on hiding them though. I used a few other peoples ideas and i hope they dont mind.