Suggest Treatment For Chronic Constipation

Hi, my granddaughter is 7 years old and has suffered from chronic constipation since birth. At the age of 2 she was sent to a pediatric gastro -interologist who put her on Mira lax. She still suffers horrible gas pain, painful bowel movements (when she can have a BM), bloated stomach, extreme sweating during a bowel movement, and also has vomiting on occasion. She loves fruits and vegetables, so fiber diet is not the problem. Please help send us in the right direction. She suffers so much pain and the doctors do not take this seriously. Kim