How dangerous is hirschsprung's disease when a child has it?

Very. Hirschsprung’s disease is the lack of nerves to the lower portion of bowel. Without these nerves the bowel cannot contract for normal bowel movements. The backup of stool can lead to infection and/or rupture of the intestines.
Infants particularly. Mortality rate for Hirschsprung's disease still ranges between 1 and 10%.Newborns and infants seem to be more likely to develop serious life-threatening complications, particularly in case of associated cardiovascular malformations.Hirschsprung-associated enterocolitis remains greatest cause of morbidity and mortality in children with Hirschsprung disease.
Depends. A lot depends on the extent of bowel affected. If ultrashort it an cause constipation. If lengthy it can result in obstruction, toxic megacolon, enterocolitis, perforation and death. It should be taken lightly.