Mr McKenna said he removed the lock from the door and it did not appear the child was in any pain.
"He was quite happy, believe it or not."
Firefighters had tried to remove the boy's finger from the detached deadlock but the digit had swollen "considerably".
The boy's parents were advised to take him to the hospital.
"We sent him to hospital with the lock on his finger - it was no biggy.
"Once the swelling had gone down it would have been easy to get off."
Mr McKenna said he didn't want to use cutting gear to get the lock off, as he would have done without hesitation had it been an adult.
"I was worried about the little fella," he said.
"I really prefer to get these things off but I was concerned with this one and thought it would be better for him to be taken to hospital."
Mr McKenna was fine about being called out.
"The parents were at the stage where they needed a bit of help and who better to call?"