What is the most likely diagnosis for a child presenting with high fever, lower abdominal pain, tenesmus, and explosive watery diarrhea that becomes mucoid and bloody?

The presentation described in the question aligns closely with a Shigella infection, which is characterized by several key clinical features. The high fever, lower abdominal pain, and tenesmus (a painful sensation of needing to pass stools) are common in shigellosis. The explosive watery diarrhea that can progress to become mucoid and bloody is particularly indicative of this infection, as Shigella typically invades the intestinal mucosa, leading to inflammation, ulceration, and, ultimately, the presence of blood and mucus in the stool.

Shigella is known for causing dysentery due to its cytotoxin effects and the resulting damage to the intestinal lining, which explains the symptoms the child is experiencing. In light of this information, the clinical scenario strongly supports a diagnosis of Shigella infection as the most probable cause of the symptoms presented.