Viral Gastroenteritis: Given the initial presentation of vomiting, nausea, fever, and slight cough, and the fact that the child was initially treated as viral, this remains a plausible diagnosis. The exposure to an exotic petting zoo could also suggest a viral etiology, although the specific link to the lizard is more suggestive of other diagnoses.

Salmonellosis: This is a common infection associated with reptiles and amphibians, including lizards. The symptoms of salmonellosis can include fever, vomiting, diarrhea, and abdominal cramps, which align with the child's presentation.
Respiratory Infection: The presence of a cough, although slight, could indicate a respiratory infection. Given the recent exposure to animals, a zoonotic infection (an infection that can be transmitted from animals to humans) should be considered.

Reptile-associated Salmonella (specifically, Salmonella Typhimurium or other serotypes): While included in other likely diagnoses, it's crucial to emphasize the importance of not missing this diagnosis due to its direct link with petting zoos and reptiles. It can lead to severe complications, including bacteremia and meningitis, especially in young children.
E. coli Infection: Though less directly linked to reptile exposure, E. coli infections can cause severe gastrointestinal symptoms and are a consideration in any case of unexplained vomiting, fever, and diarrhea, especially if there's a risk of food or water contamination.

Leptospirosis: This bacterial infection is carried by many animal species and can be transmitted through contact with contaminated water or soil. While less common, it should be considered in cases where there's been exposure to animals, especially if the child has been in contact with potentially contaminated environments.
Campylobacteriosis: Another zoonotic infection that can be associated with exposure to animals, including reptiles. It typically causes gastrointestinal symptoms and is a rare but possible diagnosis in this scenario.