In addition to its high-tech child carriers, the outdoor gear maker Kelty has jogging strollers and this useful-looking item for hikers, a diaper fanny pack. [But don't look like an ass: use a fanny pack only when hiking.]
Like Kelty's other diaper bags, this fanny pack is also designed to fit within their child carriers.
Buy a Kelty Diaper Fanny Pack at TravelingTikes.com.