Gary Foy, now aged 21, jumped a ridge on the scrambler and collided with the quad bike on which a mother was sitting with her two-year-old child.
Dublin Circuit Criminal Court heard that a medical professional who treated the child said the infant had been the most seriously injured two-year-old he had treated who had survived.
The prosecuting garda said no vehicles were permitted in the green area of the park.
Foy, of Beech Grove, Johnstown Wood, Navan, Co Meath, pleaded guilty to careless driving causing serious bodily harm at a west Dublin park, on September 17, 2020.
He further pleaded guilty to driving without insurance and driving while under the influence of an intoxicant in relation to cannabis found in his system. Foy has no previous convictions.
Judge Melanie Greally adjourned the case to October to allow a probation report be prepared.
She said it was somewhat alarming that Reilly had been riding scramblers for 10 years prior to this and that this activity had allowed to continue.
She said it was a highly risky activity and there seemed to be an "absolute inevitability" about an accident happening.
Sergeant Aidan Shaughnessy told Rónán Prendergast BL, prosecuting, that garda became aware that a child had suffered serious injuries after a scrambler collision and attended at the scene.
The child was being taken to hospital and Foy was lying on the ground a short distance from a slope when they arrived.
There was a quad bike at the scene but the scrambler was gone.
Foy, who was wearing a helmet, spoke to gardaí and told them he could not remember what happened.
He was brought to hospital and became upset when asking after the child.
He told gardaí he was riding a scrambler and then woke up on the ground. He said someone told him he had hit someone on a quad.
He did not know where the bike he was riding had gone.
The baby's mother described how she had been sitting on the stationary quad with her son, who had a helmet on before a scrambler came over the hill.
She said it came toward them and she tried to grab the baby but did not remember much else.
Foy, who was then 19 years old, was interviewed and said he was familiar with the park and had been riding a bike for 10 years.
He said there was a "set route" with a jump over a ridge. He said he had gone that route hundreds or thousands of times and considered himself a good driver.
He told gardaí that you could not see what was on the other side of the ridge but "everyone knows" there were bikes coming over it.
He said he had asked for a go on someone else's bike and went for a jump. He said he had been going 20 to 25 kph.
He did not remember what happened and next thing he was going to hospital.
The court heard the child was seriously injured and suffered a traumatic brain injury.
He has been doing well since his discharge but will require rehabilitative services.
A medical professional said that the child had been the most severely injured two-year-old they treated that had survived to date.
Mr Staunton asked the court to take into account letters from a youth workers outlining his engagement with relevant services, as well as his deep regret and remorse for the incident.