Viral Gastroenteritis: This is the most likely diagnosis given the symptoms of fever, runny nose, and diarrhea in a 1-year-old child. The low WCC could be indicative of a viral infection, which often presents with leukopenia.

Respiratory Syncytial Virus (RSV) Infection: RSV can cause symptoms similar to those described, including fever and runny nose, and is common in young children. Diarrhea can sometimes accompany RSV infection.
Influenza: Although more commonly associated with respiratory symptoms, influenza can also cause gastrointestinal symptoms like diarrhea, especially in children.
Adenovirus Infection: Adenoviruses can cause a range of symptoms including fever, runny nose, and diarrhea, making them a plausible diagnosis.

Bacterial Sepsis: Although the low WCC might suggest a viral etiology, bacterial sepsis can present with leukopenia, especially in young children. It's crucial to consider and rule out bacterial sepsis due to its high mortality rate if left untreated.
Meningitis: Fever and irritability could be signs of meningitis, a serious and potentially life-threatening condition. Although diarrhea and runny nose are not typical symptoms, it's essential to consider meningitis in the differential diagnosis of a febrile child.
Urinary Tract Infection (UTI): UTIs can present with nonspecific symptoms like fever in young children and can lead to serious complications if not treated promptly.

Immunodeficiency: A child with recurrent or severe infections might have an underlying immunodeficiency. The low WCC could be a clue, although it's more likely related to the current viral infection.
Leukemia: Although extremely rare and unlikely, leukemia can present with fever, infection, and abnormalities in the white blood cell count. It would be an unusual cause of these symptoms in a 1-year-old but should be considered if other diagnoses are ruled out and the condition persists or worsens.