My heart goes out to your daughter...my grandon also has GERD at 8 mo, so I understand your frustrations...it's just awful when children get sick.
I have severe GERD and I've never experienced the symptoms you mention but GERD can have a range of symptoms, mild to severe. It wouldn't be the right location at the bellybutton. My thought is that if the Prevacid is not helping, it's possible that it's something else and it was misdiagnosed since the endoscopy didn't show anything. Also, another thought that sounds wierd but it happens sometimes, is that the cramping and nausea started after the appendectomy. She had nausea before, but that was probably the appendix inflamation before it ruptured. But now after the surgery, she has cramping, nausea and vomiting correct. Well, what I'm getting at is that sometimes doctors have been known to leave a sponge or gauze inside after surgeries, which could account for the symptoms.
I mention this, because it happened to an aunt of mine...the doctor left a sponge inside her after surgery and she got sick and nauseous from it.
This is probably not what it is but I mentioned it because of my aunt. Another possibility is that some of these symptoms can be caused by emotions. We often underestimate the power that our mind and feelings have over our bodies. Stress can shut down many of our bodies systems, causing cramping, headaches, dizziness, nausea etc. and it's the last thing we think of. I know that stress stopped my periods for 3 months one time and 15 months another time. There was nothing physically wrong with me...the stress just shut down my endometrial system.
I truly hope you find the answer soon for both your sakes...my prayers are with you.