My Niece Is 2 She Has Had A High Fever

My niece is 2 she has had a high fever reaching up to 103.6 without medicine. This fever has lasted 7 days as of today. She has complained about her mouth hurting and head hurting. She has been brought to our local er 2 times now with a finale diagnosis of pharyngitis. She does have the white tounge as of yesterday so i assume thrush. She now has "fever blisters" in and around her mouth now. Flu and rapid strep came back negative. They sent her swab for strep off to get cultured. I am looking for a better diagnosis. Please help us!