If this were my kid, I would ask the pedi for a referral to a pediatric gastroenterologist, and get a colon
X-ray. If there is a delay for this kind of assistance, I would also start her on a gentle non-medicinal stool softener like Benefiber, every day. And stick with it for a long time. Once her colon is distended, it can hold a lot of stool, which gets terribly impacted. Sometimes the only way to get it out is at the hospital.