Rhys Atkinson made the comments during a class about the refugee crisis in Syria at St Michael and All Angels Catholic Primary School in Upton.
His parents were called in to see the head at the Wirral school, who alerted social services about the comments, according to The Daily Mail.
The boy's mum and dad, who are separated, said it was "idiotic" for teachers to jump to conclusions.
Rhys' father Mark Atkinson said that his son had asked about the crisis and he had explained how it had begun: "The next day he wrote in class that he wanted to fight terrorists - it was just a childish thing and he didn't mean anything by it."
"Then, all of a sudden, it's being implied by his teachers that he is one.
"I was called in for a meeting with the head and couldn't believe what she was telling me.
"How could the school believe my eight-year-old child could be a terrorist?
"He didn't say he wanted to be a terrorist, he said he wanted to fight them.
"It was an overreaction and completely inappropriate."
His mother, Louise Aspinall, said: "They should react to a young child's naive comments like adults and not jump to crazy conclusions.
"It is idiotic and completely out of order."
Rhys wrote that "I want to go to Syria and fight terrorists."
Under the Government counter-terrorism strategy, teachers are encouraged to report any pupils they fear are in danger of being groomed by terror groups.
The school declined to comment.