They don't , this is their underwear once they land on earth.

They could use adult size nappies?

Great question! Here is some information from the "Ask an Astrophysicist" feature on the NASA website...   Each Space Shuttle has a toilet that can be used by both men and women. Designed to be as much as possible like those on Earth, the units use flowing air instead of water to move waste through the system. Solid wastes are compressed and stored on-board, and then removed after landing. Waste water is vented to space, although future systems may recycle it. The air is filtered to remove odor and bacteria and then returned to the cabin. Astronauts brush their teeth just like they do on Earth. There is no shower on the Shuttle, so astronauts must make do with sponge baths until they return home. The toilet that was first flown aboard STS-54 is completely new in design and offers new and improved features: The new toilet features better hygiene, larger storage capacity, greater dependability, and an overall cost savings in maintenance. -The previous model had a 14-day capacity for storage of waste material. The new model has an unlimited storage capacity. -The new model features a cylinder system where a plastic bag is placed in the toilet before use. The bag is then sealed and is forced to the bottom of the cylinder after each use by a plunger attached to a lever. A new bag is then placed in the toilet for the next astronaut. When the cylinder is filled, it is replaced by a new cylinder. -The previous model relied on air flow to pull the waste to a holding tank. None of the waste was separated as it is now. The new system provides better hygiene conditions. There was no way to empty the old system. When it was full, it simply could hold no more waste materials. It had a 14 day capacity. -The new toilet also provides an odor-free environment. The old model did not. -The opening in the lid of the toilet was increased from 4" to 8", allowing for easier handling of the plastic storage bags. -The urine collection system was also improved. A newer type of fan system is being used to force the urine to a holding tank where it is periodically ejected into space, where it vaporizes. -The previous system had trouble with corrosion in the fan system. -The new toilet can be cleaned without removal from the orbiter at the completion of the mission, reducing the cost of servicing. -The previous system must be removed and sent to a company in Houston, Texas for servicing.   http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970411a.html   Interesting, isnt it?