Final answer:
The assessment finding that should be reported immediately if observed in a child with meningitis is a petechial rash, which is a unique sign of meningococcal meningitis. This rash is a response to virulence factors that disrupt blood vessels, causing blood to leak into surrounding tissues.
Step-by-step explanation:
The assessment finding that should be reported immediately if observed in a child with meningitis is a petechial rash. A petechial rash is a unique sign of meningococcal meningitis, characterized by tiny, red, flat, hemorrhagic lesions on the skin or mucous membranes. This rash is a response to virulence factors that disrupt blood vessels, causing blood to leak into surrounding tissues. The formation of larger lesions can indicate the progression of the infection and the risk of shock and organ failure.