John BoydBoyd's The Organ Bank Farm.
The kids concerned are mentally or emotionally disturbed, and liable to be stripped down for spare parts if they can't be cured in time. This particular child is being cured of incontinence by simulating A nappy rash for number one, a corncob for number two"A nappy rash for number one, a corncob for number two". It also takes place in a world where a virulent plague has reduced the population to about ten percent of previous.