"She wasn't progressing as she was meant to, people said she'd catch up, but I knew something was wrong," she said.
"We started genetic testing and everything was coming back normal, we received the Rett diagnosis in March this year."
Although it was the diagnosis she expected, she said it was the last one she wanted.
"It was really upsetting when we found out what it really was.
"There is no cure. There are only treatments to help with the symptoms."
Kendyll has experienced difficulty learning anything, starting to crawl at 2.
"She's lost all purposeful hand movement, she will never walk or talk, she can't tell us her own needs," Miss King said.
"Every day is impacted by it."
Steph King wants to help find a cure for Rett Syndrome and help her 3-year-old daughter Kendyll Stewart. Photo/Stephen Parker Kendyll has already begun suffering small seizures, but Miss King know those will likely get worse.
Her symptoms will likely grow to include scoliosis, balance problems, difficulty breathing and severe seizures.
But for now Kendyll is a happy 3-year-old who laughs regularly and loves spending time with people, including her little brother.
"We just deal with everything as it comes," Miss King said.
Their current focus is on helping Kendyll to communicate and they are on the waiting list for a machine that will allow her to speak using her eyes.
"Not knowing her needs is really tough. When she is screaming we don't know how to help her.
"Rett girls are often really smart girls, they are just trapped in bodies that don't work."
When a mutual friend approached Miss King's partner wanting to raise money for the family they wanted to make sure they were helping others.
"We thought it would be nicer to benefit other girls as well, so the money will be going to the Rett Research Trust," she said.
The fundraiser, organised by Limitless Car Scene, is an opportunity to raise awareness for the syndrome.
The Pop a set for Kendyll! Facebook page says "gather ya loose change up and come for a day out of skids".
A tyre machine and compressor will be available on sight for people using the skid pad
"We're really looking forward to it actually," Miss King said.
"It's a chance to bring the scene together to do what they do best."
For more information or to donate to Rett research visit www.girlpower2cure.org .
What: Pop a Set for KendyllWhere: Skid pad, North of Rotorua on SH5, there will be purple balloons at the gateWhen: Saturday, November 4, from 10amCost: $30 for unlimited access to the pad, $10 for passenger band, $5 for spectators