Constipation is an abnormal difficulty with passing bowel movements (bm's). In children it also involves withholding bm's. Mild and brief spells of constipation happen to all children sometimes. Usually this normal constipation is caused by a change in diet or a viral illness.
More serious constipation has many symptoms. These usually include relatively infrequent bm's, passage of small hard bm's, and pain with bm's. Sometimes only your child knows about these symptoms. So ask your child. Also you may or may not notice your child holding their stools in. Their face may be red, legs clenched and they may go to a quiet place such as a closet or corner. It may look as if your child is having trouble passing a bm, but more likely they are holding it in for fear of the pain. Other symptoms may include: "stomach" pain, soiling in underwear, enuresis, urinary tract infections, vomiting, loss of appetite, and bloating.
If your child has these symptoms, call your pediatrician for an evaluation.