My daughter was diagnosed in 2nd grade with Celiac Disease, after spending her first seven years of life with extreme stomach pain. Initially they called it Colic. Of course, she never grew out of it. Each year became progressively worse. Pediatricians kept calling it "growing pains", and many suggested to us that she was a hypochondriac. (We switched pediatricians several times because of our dissatisfaction with them.) Her behavior problems were so extreme, we took her to psychiatrists and psychologists who thought she was bi-polar (as a pre-schooler!) One wanted us to put her on a very strong anti-psychotic drug, but we refused. Finally, we insisted on being referred to a gastrointerolgist who suggested the endoscopy. In the meantime, her current pediatrician was telling us it would be a waste of time. She suspected her pain and diarreah was due to stress at home.
It turns out her villi were almost completely sanded down. Since 2006, after her diagnosis, we put her on the gluten-free diet. As long as we don't have cross-contamination issues (restaurants or misguided pharmacists), she is a completely different and NORMAL child. If she does get gluten in her system, the symptoms start up like clock-work, 12 to 24 hours later, and last 1 to 2 weeks.
We no longer take our kids to pediatricians (even though they are now 11 and 9). We all see the same family practioner who seems to be well-versed on gluten intolerance.