It is very possible that your son and his daughter have condition known as Hirschsprung's disease or megacolon.
It is a congenital disease (born with) that is usually passed from parents to the children.
Actually, a segment of baby's colon (large bowel) misses nerve cells in wall muscle, interrupting the signal from the brain and prevents peristalsis(muscle contractions that move the food) in that segment of the intestine.
This results in severe case of constipation, painful defecation with blood in stool due to anal tears when the hard, large stool is passed, or/and because of hemorrhoids.
This condition often needs to be treated because in more severe cases can lead to serious complications!
Consult gastroenterologist and pediatrician specialized in abdominal surgery!
Best wishes!
Marija