Is she able to retain water?
Since how long are her symptoms?
Has she diarrhea, pain abdomen, bloating, nausea/dry retching?
You can try dimenhydrinate children's liquid. This can be just stomach irritation. Signs of dehydration like excess thirst, fast pulse / heart rate, dry tongue would warrant an examination at ER. Projectile (shooting) vomiting, fever, persistence of the vomiting and inability to keep even water / Gatorade also will point for an examination by a doctor.
Please feel free for your follow up questions.