"So we did that and the headaches went away and about a day before he was actually diagnosed he started displaying more symptoms. He went to school and he had an episode. We don't really know what type of episode, [the school] reckon it was like a seizure and he went funny and couldn't walk."
Ms Clark took him to Middlemore Hospital and he again couldn't walk.
"We were like, 'what is going on here?' Middlemore Hospital said a CT scan is quite abrasive for a kid ... and thought he was acting out so the hospital referred him to their psychology team."
The next night he played chess with his father, Matt Stone, and when Sean couldn't reach out for pieces, his parents knew there was something wrong.
He was driven back to Middlemore Hospital where the family demanded a CT scan and the lump was finally revealed. After surgery of about nine hours the doctors extracted the large tumour.
"It was bigger than a tennis ball, even after the surgery all the surgeons came to check on him and said it was the most impressive tumour they had ever seen and they were sort of preparing us for the worst."
But after many months of chemotherapy and 31 rounds of radiation, Sean, Ms Clark, his stepfather Anthony Baker and friends Christian Parry and Christin Hart were donning superhero T-shirts and taking part in tomorrow's Round the Bays run.
Sean wanted to raise money for the Child Cancer Foundation after earlier raising money at his school, Reremoana School in Manurewa, for Ward 27 at Starship hospital.
"He's right into helping people out," Ms Clark said. "He's very polite for his age and he gives a lot, it's his chance to give back."
The family aren't expecting the run to be easy for Sean as he still loses his balance when he runs, loses co-ordination in his right-hand side, walks with a limp and can't close one eye properly.
It has been eight months since radiation treatment finished and there is still no sign of any cancer cells.
He is due for his next check-up at the end of this month. Although it has stopped him from participating in his favourite sports - karate and soccer - or riding his bike, his family are forever hopeful.
"He was a very, very active young boy," Ms Clark said. "Now it will be something that we will work towards but it's probably not going to happen for a bit of time."