Have they got a water infection? Strong smelling urine and frequent accidents in a previously dry child would be common symptoms For the laundry, soak in bicarb solution before you put in the main wash
In terms of the laundry itself, soak/knead each piece of wet clothing in cold water then wring out, do this a few times so that most, if not all, of the urine goes. Should smell much better already. Then continue to wash as usual. I'd opt for a 40C wash (in case you're using a 30C cycle). Also, with smellier things I've found that using prewash (with powder in the prewash & main wash sections of the drawer) very good. Another trick is using a pleasant smelling disinfectant liquid like this in the soft rinse section of the drawer. Haven't had to do this when I rinse out urine soaked clothes first.