Disease of infants and young children that makes breathing difficult (5)
I believe the answer is:
croup
(Other definitions for croup that I've seen before include "Children's ailment with hard cough and difficulty in breathing" , "Inflammation of the larynx and trachea in children" , "Children's disease with hard cough and wheezing" , "Rump of a horse" , "Throat condition with a hacking cough" .)