A lot of people on here make liners out of fleece. When you cut it to the size of the cage, the ends don't unravel, so they're very easy to make, especially if you only want one layer. If you use lighter colored liners (Lily has cream colored liners in her cage) it'll be very easy to see where your hedgehog is peed. There's also the added bonus of being able to easily check if the pee is off-colored or has blood in it.
The liners, unless you change every few days, will also start smelling like urine, so your folks may continue to notice it. I've read that some people place small bowls of baking soda or something out to help absorb the smell, or you can also use an air purifier if it gets too bad. To wash the liners, I wash them with Tide-Free detergent, which is scentless and dye-free. I know other people also do a rinse with vinegar to help take the smell out. Make sure you don't use a scented dryer sheet too.