What is causing the lower back pain in my 6 yr old dealing with costipation?

Bowel or urinary : In chronic constipation children often have associated urinary tract problems which can cause back pain. The child should be screened for urinary tract infection as well as being put on a bowel retraining regime, ie stool softeners and enemas as needed.

Referred pain: It is most likely referred pain from the abdominal pressure or bowel distention related to the constipation. We can't differentiate pain well between inside our abdomen and out so we often have referred pain that we perceive as other pain. It should go away with relief of constipation, but if it doesn't, then see your pediatrician. I hope that helps and that your little one feels better!