The 39-year-old faced a hearing in the ACT Magistrates Court on Wednesday, after pleading not guilty to one count of common assault.

Prosecutors had alleged he used the stick to hit the 8-year-old on the leg so hard it left a welt at the Gordon Ponds party on October 23.

On Wednesday he took the stand to deny the allegation.

He said the stick, which was made from cardboard and paper, had been bent and so he had been helping his daughter to use it to hit the piñata.

The cake-shaped piñata had broken and scattered lollies across the ground, prompting a rush from some of the 20 children at the party. One boy had stepped on his daughter's hand in the ensuing commotion.

He had picked up the boy, who had fallen on top of his daughter, and moved him to the side before retrieving his crying daughter.

"I just dropped [the stick] and grabbed my daughter," he said.

He denied any allegation he had hit the 8-year-old.

He said the boy's father had confronted him and accused him of hitting the boy. He said they had sworn at each other before the father "threw" his son in the car and drove off doing a burnout.

A friend of the man also gave evidence, and said he had seen the incident and the man pick up his daughter but not strike the boy.

In closing submissions, the man's defence solicitor Jacob Robertson said there was no question the boy had suffered an injury.

"The question is how that occurred," he said.

There was evidence the father smacked the boy after he behaved aggressively towards others kids on the jumping castle, he said.

He said it was clear there was animosity between the two fathers.

At the end of the hearing on Wednesday, Magistrate Karen Fryar said she did not find the charge proved beyond reasonable doubt.

She said she was not satisfied about what had happened given the conflicting accounts of the day. That left her with the large red welt on the back of the boy's leg and how it occurred. But she said she did not disbelieve the man's evidence, and so she dismissed the charge.