Viral Gastroenteritis: This is the most likely diagnosis given the patient's symptoms of high-grade fever, watery non-bilious vomiting, general body weakness, headache, stomachache, and loss of appetite. The absence of cough, colds, diarrhea, and altered sensorium, along with the remittent nature of the fever, supports this diagnosis. Viral gastroenteritis is common in children and can cause significant dehydration, which is consistent with the patient's moderate dehydration upon admission.

Dengue Fever: Given the patient's residence in Quezon City, Philippines, where dengue fever is endemic, this diagnosis should be considered. Symptoms such as high-grade fever, headache, general body weakness, and loss of appetite are consistent with dengue fever. However, the absence of specific symptoms like petechiae, bleeding manifestations, or a more pronounced thrombocytopenia makes it less likely than viral gastroenteritis.
Influenza: Although the patient does not have cough or colds, influenza can present with high fever, headache, and body weakness. The recent onset and the presence of vomiting could also fit with influenza, especially if it's a gastrointestinal presentation in a child.
Food Poisoning: This could be a consideration given the vomiting and stomachache. However, the high-grade fever and general body weakness might suggest a more systemic infection rather than a simple food poisoning.

Meningitis: Although the patient does not have altered sensorium, which is a common symptom of meningitis, the presence of high-grade fever, headache, and vomiting necessitates consideration of this potentially life-threatening condition. Meningitis can present subtly in children, and missing this diagnosis could have severe consequences.
Septicemia: The high-grade fever, general body weakness, and loss of appetite could also suggest septicemia, especially if the patient has a bacterial infection. Prompt recognition and treatment of septicemia are crucial to prevent serious complications.
Typhoid Fever: Given the patient's symptoms and geographical location, typhoid fever should be considered, especially if the patient has been exposed to contaminated food or water. Typhoid fever can present with fever, headache, and abdominal pain, and it requires specific antibiotic treatment.

Brain Tumor or Abscess: Although very rare and less likely given the acute onset, a brain tumor or abscess could potentially cause symptoms like headache, vomiting, and general body weakness. However, these conditions would typically have a more subacute or chronic presentation.
Hemorrhagic Fever: Given the geographical location, hemorrhagic fevers like Ebola or Marburg (though extremely rare in the Philippines) could be considered in the differential diagnosis due to their severe presentation with fever, vomiting, and potential for hemorrhage. However, the lack of exposure history and the specific symptoms make this less likely.