The two-year-old from Somerset tripped over a stair gate and fell onto the pencil which she was carrying. It bounced off her eyeball, just missing three major blood vessels, but jamming 1.5 inches into her brain.
Neurosurgeons operated on her for four hours, removing a section of her skull to remove the pencil.
Thankfully, she has made a full recovery from her dramatic accident.
Her father Martyn, 34, a professional model maker who is married to Michelle, said: "The pencil missed her eye completely as it bounced off the top of it, and we have been told there was no damage to the optic nerve - which is remarkable.
"The stair gate was there to keep her safe, but as she tripped over it while carrying the pencil she fell onto it.
"If anything happens to your child you are shocked. A broken bone would be bad enough, but something happening to the eye, head or brain is one of the worst things that could possibly go wrong.
"Fortunately my wife, who's a nursery nurse, kept a level-head and realised not to try and get the pencil out."
The little girl had to take anti-seizure drugs as a precaution due to the brain injury - but remarkably has been fine since the operation.
"The pencil was stuck so hard that they had to pull part of her face off and take out part of her skull to take out the pencil," says Martyn. "They then put Wren's skull back together with plastic plates and screws, which will biodegrade."
Frenchay consultant neurosurgeon Ian Pople, who operated on Wren, said she was 'incredibly lucky', adding others who suffered similar injuries had suffered permanent brain damage or even died:
"The pencil was within a millimetre of hitting a big blood vessel in the brain. She was incredibly lucky as she came out, fortunately, with no major bleeding. It just skirted the top of the eye and that it didn't damage the eyeball itself was very fortunate.
"She was very lucky not to have suffered any permanent damage as far as we can see. I have also known of a case, which I did not deal with myself, that was fatal. And another, which was not fatal but caused a lot of damage because it hit the blood vessel."
Martyn has signed up for a first aid course since the incident, and is now raising money for Frenchay's children's unit to provide more toys to entertain young patients, taking part in a cycle ride from Stratton-on-the-Fosse, in Somerset, to Weymouth.
To sponsor the team's efforts for Headway Bristol visit Justgiving.org.uk/ride4recovery.