I usually buy things two at a time to have a back up. I just hate running out of that stuff. I get body wash, shampoo, toothpaste and tooth brushes. My littles still sometimes bites hers and it's amazing at how quickly she can go through them! I also have floss on hand for them...the kid flossers, but I generally just keep a bag of those on hand instead of back up stock. My SIL would add detangler spray to that list, her kids (even her three year old) have gorgeous, long, thick hair. She'd also have body lotion, every once in a while, she lotions her kids up after bed time.
First aids stuff is kept in a locked fishing tackle box on a high shelf. We keep ours well stocked, but I don't buy back ups of things for that. I leave the empty containers on the counter in our kitchen to remind me we need more. Fever reducer, benedryll, bandaids, a few larger bandages, liquid skin, tweezer, nail clippers, thermometer, sun burn ointment, cold medicine, ear drops to get out water, orajel, neosporin, ointments and an ear scope are some of the things we have in ours.