That has improved the life of the fair-haired boy, who says he wants to be a Formula One racing driver when he grows up.
But the weak, enlarged heart has caused numerous medical crises and left him severely under-sized, and is now nearly worn out.
Kaden is on heart medications and without a transplant he is expected to gradually deteriorate, then die.
"They talked about having around 12 months' expectancy from here, give or take six or 12 months," said Mr Probst, an IT manager who has taken leave without pay to spend several weeks at home with his son, daughters aged 2 and 4 and wife Jody.
Dr Ruygrok said how long people waited for a heart transplant varied partly because the organs had to be matched by blood group and patient weight. "It depends on an appropriate donor, and the donor always dies under tragic and sudden circumstances. It could be a wait of two days or up to a year. It's a random, tragic event.
"I'm guessing that Kaden may have to wait three to six months."
Dr Ruygrok said a youngster lighter than 15kg needing a transplant would require a child's heart and would be referred to the Royal Children's Hospital in Melbourne because New Zealand had few child organ donors. Patients weighing 15kg or more could receive the heart of a light adult.
Even with a successful transplant, Kaden's life expectancy is limited. Dr Ruygrok said most recipients were expected to live more than 12 years and with modern anti-rejection treatment could live significantly longer.
Kaden is tiny for his age, little taller than 2-year-old sister Hannah and slightly shorter than 4-year-old Aalia. He weighs a little over 15kg - 8kg less than the average for his age.
He has little appetite, sometimes can't keep food down and lacks energy. His failure to thrive is because his heart pumps only one-third the amount of blood of a normal heart, causing the blood pressure to rise in veins draining into the heart.
Paediatric cardiologist Dr Tom Gentles explains: "The 'back pressure', if you like, gets distributed to all the veins in the liver and your tummy and that makes you feel a bit uncomfortable and interferes with your appetite and your general sense of wellbeing."
Mr Probst said Kaden led a "relatively normal" life. He rarely missed a day at school and he loved practising his swing at the Pupuke Golf Club.
Until he was nearly 4, when he had treatment to control his heart's rapid rhythms, life was hectic. His parents sometimes had to dunk him in ice to shock his heart back to a normal rhythm and when that didn't work, rush him to hospital.
In October, while in the Starship's heart unit having a procedure to measure his heart for a transplant, Kaden had a cardiac arrest.
"They said there was no heart-beat for 10 or 15 minutes," Mr Probst said. "They did a hell of a job resuscitating him."
YOU CAN HELP
To become an organ donor go to www.donor.co.nz
HEART TRANSPLANTS
* Done in New Zealand only at Auckland City Hospital.
* 10-12 performed a year.
* Up to 3 patients a year die waiting, because of shortage of organ donors.
* Cost: $120,000 to $150,000 for transplant, after-care and ongoing anti-rejection drugs.
