Final answer:
Neck stiffness in a child post-seizure is the most concerning sign and could be indicative of meningitis, which requires prompt medical attention.
Step-by-step explanation:
When assessing a child who experienced a seizure, the assessment finding that should concern the EMT the most is neck stiffness. Neck stiffness can be indicative of meningitis, which is a potentially life-threatening condition if not treated promptly. Symptoms such as seizures, bulging fontanel, and opisthotonos may also accompany meningitis. Tachycardia, high fever, and a short postictal phase may be concerning as well but are less specific and may be seen in a variety of less serious conditions.