Oh Rainbow your poor DD that's awful for a child to cope with. DD2 has a degree of hypermobility and apparently that can cause acid reflux as muscles that should keep the stomach "closed" are too weak to do their job. But nothing like your DD is suffering 
Interested to see this thread as I've been told I have GERD/gastritis and have been on lansoprazole for 15 months without success. I was prescribed it as I could feel something moving in my stomach and had a pain like a stitch in my ribs, was waking up in night to poo and had trouble passing urine. My GP said oh yes this is indigestion which is why you are getting a lot of acid, and I said no I am not getting acid and she said oh yes you are ...!!
Before this, although I'd had ulcers some 15 years previously, I rarely had indigestion or "acid" but since I've been on lansoprazole I've developed a rasping pain in my trachea when I make any sudden movement and had about half a dozen episodes of acid beyond all acid - literally comes into my throat like smoke - very odd - but only these few instances, you all seem to have symptoms pretty much all the time, affected by different foods. I don't seem to have any of the symptoms you are all describing, it sounds awful.
So I think the lansoprazole is causing over-production of acid. And 15 months on I have the same "feeling" in my stomach/ribs. I've had endoscopy, colonoscopy, bone scan and blood tests, seen two gastroenterologists and a rheumatologist, no luck. My other symptoms are dry flushing - not menopausal hot flush - but literally looks like someone has stamped blocks of purple on my face, neck, chest and breasts and awful aching in the armpits and sometimes all down my arms.
So I don't think its GERD, but its the GPs' favourite idea!