Get a non-chlorine based bleach (like Oxicleanse or Napisan). These will also help bleach fabrics, but without leaving any of the annoying yellow patches the way chlorine-based bleaches do. Wash the garments according to the instructions on the Oxicleanse or Napisan box. Then hang the items in the sun to dry. The sun exposure is key. The brighter the sun and the longer the item gets exposed to sunshine, the better. The sunshine will (eventually) bleach the items white again, and bleach out any urine stains. If at all possible, wash items only with like colours (i.e., only wash whites with whites). This will also help prevent whites going grey. Sodium bicarbonate can be used instead of Oxicleanse, but is not as effective (better at removing odours than bleaching out stains, really). Exposing the stained clothes to a bright light indoors won't be nearly as effective as sunshine, alas. You just will have to get your undies high up on the line for the neighbours to view, if you really want them white again.