"Don't move!" I screamed.
For a long time he didn't. Then at last, from under his crash helmet, a pair of blue eyes blinked and brimmed with tears.
"Da-ad!" he wailed.
"Where does it hurt?" I asked.
He didn't need to answer because I could see for myself. I was grateful he was wearing his helmet and elbow pads. But, as well as the cuts, grazes and bruises, there were wounds I could not see.
Over the next few days he was quiet. He didn't want to go out to play. He was unusually moody.
"What's up?" I asked.
"Nothing," he replied.
And I knew what about. He'd had a bad shock and now the memory of it was replaying over and over in his mind like a horror film.
"I was really scared," he told me eventually. "I don't ever want to be that scared again."
We all want to protect our kids from life's sharp corners and at that moment I wished for a magic pill to wipe away the bad memory and restore my lad to his former happy-go-lucky self.
In fact, over the years, I've often wished for a memory-wiping potion. Just imagine all the heart ache it would save!
It reminds me of that movie The Eternal Sunshine of the Spotless Mind, which I watched with my wife the other day. There's a scene where Kate Winslet gets over her ex-boyfriend (played by Jim Carrey) by having her memories of him wiped from her mind.
But I wouldn't stop there.
My football team getting beaten to the title in the last minute of last season? No problem. There's always this season.
A fly the size of a Boeing flying near my soup? No problem: they are no longer the single most irritating thing that has ever bothered me any more.
Trip to the doctor? No problem, because I'd have forgotten the last time I went two years ago he told me to cut down drinking?
Arguments with my wife? They'd never happen because I'd keep scratching my brain for her excuses at never taking time off work to give me a rest from the kids.
Heck, I might even choose to forget I had kids! Cry freeeeeeeeeeedommmmmmmm!
After all, we can now nip and tuck other parts of our existence that we're not so keen on – even men are having their Moobs removed nowadays - so why not take a metaphorical scalpel to our memories?
It seems scientists may be on the verge of doing just that. A couple of years ago, researchers claimed to have found a way to erase bad memories by using 'beta-blocker' drugs, which are usually prescribed to patients with heart disease.
The aim is to use these drugs to help treat patients with serious post-traumatic stress disorders.
But in the future who knows what memories we might choose to obliterate?
In the same way that we can now choose to have smaller Moobs and bigger, er, lady impressers, we could in theory all live a life filled with only happy memories.
My first thought when I read this news was: Okay, where do I sign up?
But then I remembered my mum. She was diagnosed with Alzheimer's Disease when she was 70 and died two years ago. She would have been 77 this week.
Amongst other things, illness strips the sufferer of their memories. Shock defeats, dark phobias and doctors may all fade from the mind of the Alzheimer's patient. But they are not the only memories to go.
Along with them go holidays, birthdays and Christmases. Husbands and wives are forgotten. Children become strangers. Gradually, inexorably, the past disappears.
It is the most heartbreaking illness because it steals a person away while they are still alive.
I tried many times to imagine what my mother was going through. I tried to imagine how it would be not to be able to remember my wedding day, the births of my children, the laughs I'd shared with my family, the tears we'd shed together over a tragedy.
I tried to imagine it and I simply could not.
After all, how do you imagine the extinction of your own identity? That's what our memories are – they are us. Good and bad, they make us who we are.
They connect us to our loved ones. They are the stories we tell, the moments we share and the lessons we have to learn - often the hard way.
When we are little, memories teach us not to touch a hot pan, to be careful on the stairs and to use our brakes when we're going too fast downhill on our bikes!
As we grow, they help us negotiate life's ups and downs, they warn us not to repeat our mistakes and they spur us on to overcome adversity.
In any case, it is often impossible to separate bad memories from good.
When my friend Debbie thinks of her sister, she remembers her smile, the way she always brushed her hair 100 times before bed and the fact that she was wearing red jeans when she was hit by a car and killed.
Even the most horrific of memories has a purpose.
Ultimately, though, bad memories are our way of appreciating the good things in life.
At the end of The Eternal Sunshine of the Spotless Mind, Jim Carrey and Kate Winslet's characters realise that in forgetting the bad things about their relationship they have also cast aside the good things.
Eternal sunshine, they discover, is meaningless. You only appreciate the sunshine when there are a few black clouds on the horizon. We all face black clouds in our lives.
But how many of us appreciate them for the good they bring?
The other day I was talking to a friend of mine. As a child, she was regularly beaten by her father. She makes a point of remembering the details.
I asked her: "Why do you want to remember something so terrible?"
She replied: "So I never forget how lucky I am now."
The eight-year-old is back on his bike. He's taking it slowly, especially on the big hill.
"Don't forget those brakes!" I keep telling him. "I won't!" he says.
And I know he means it.