Liam Whoriskey, 25, was found guilty of killing the three-year-old by a unanimous verdict by the jury of six men and five women at Derry Crown Court.
The former waiter, of Glenabbey Gardens in the city, was also found guilty of a single count of child cruelty against Kayden.
Whoriskey shook his head as the guilty verdicts were read out by the jury foreman in Court 4 at Bishops Street Courthouse after an 18-day trial.
Kayden was found dead in his mother’s flat in Colmcille Court in the Bogside at 10 am on Sunday, September 17, 2017.
Erin McLaughlin had left Whoriskey to babysit her two children while she went out socialising with friends on Saturday, September 16.
Speaking after the jury delivered its verdict following 80 minutes of deliberation, Erin McLaughlin said:
“It has been a very, very difficult time,” she told reporters.
“I am happy with the outcome and I hope he rots in jail.
“Nobody can ever understand how difficult it is to lose a child in this way.”
Her son died as a result of fifteen blunt trauma injuries which caused a bleeding and a swelling of his brain.
He sustained the injuries while he was in the sole care of Whoriskey.
Judge Philip Babington excused the 11 jurors for life and said he would sentence Whoriskey following a plea hearing on Thursday, November 29.
There was a noticeable police presence as both families traded insults on the way out.
Child killer Whoriskey was taken away by prison staff back to the top security Maghaberry prison.
After today’s verdict, a spokesperson for NSPCC NI said that Kayden was “subjected to unimaginable pain and anguish during his short life – with devastating consequences”.
“Young children, depend entirely on those caring for them, so it is vital that anyone concerned about their welfare speaks out.
“They can call the NSPCC Helpline on 0808 800 5000 where trained counsellors will be there to listen and potentially take action on their behalf,” the spokesperson said.
The NSPCC’s confidential helpline is available 24/7 on 0808 8005000 or by emailing email@example.com.