You should take your child to the ER for not pooping if your child has not pooped within 1 to 2 weeks or if your child has any other symptoms like vomiting including green vomiting, or rectal bleeding or abdominal pain, especially if the abdominal pain is persistent.
You should seek immediate medical attention and take your child to the ER if your child has any severe or constant abdominal pain or is vomiting, especially if the vomiting has blood in it or is green in color.
Also if the child has any bright red blood or dark/tarry blood in their poop or on the toilet paper or the child has a swollen or distended abdomen that is tender to the touch or the child looks or acts very ill or the child has been straining or if the child is in pain for more than an hour after an enema or suppository you should take the child to the ER.
It's normal for a child to go a few days or so without pooping, but if the child is not pooping within a week or longer it's often a sign of constipation.
If the child's constipation lats longer than 2 weeks, the child is bleeding from their anus, the child has painful bowel movements or the child has a change in their appetite or they are losing weight or they have any fecal soiling of their pants the child should see a doctor or go to the ER.
You can try giving the child some prune juice or laxatives and even stool softeners to help the child poop.
But if the child still is not pooping you should seek medical attention to be safe.
The child might have an impact bowel and need it cleared up by the doctor.
Increase the child's fiber intake, such as through eating, oatmeal, bread, cereals, beans, apples, pears, prunes etc which can all help a child poop and ensure the child is staying hydrated by drinking enough water.