Charges against a friend of the mother also were dismissed.
"At the end of the day, or in this case at the end of a trial, if found guilty, there is no punishment, be it a fine and/or jail sentence, that I could impose that could possibly be as harsh as the punishment (the two) have inflicted upon themselves," Justice David Truesdale said in granting the defence attorneys' motion Tuesday evening.
Paula Montgomery, 49, and Peter Cooper, 51, both of Orillia, Ont., were charged with endangering the welfare of a child and providing alcohol to a minor after Alexander Montgomery drowned behind Old Fort Niagara in September 2011 during a War of 1812 weekend encampment at the historic fort.
Authorities said the teen drank beer as participants sat around an evening campfire and apparently later fell off a breakwater as the others slept.
Tests showed his blood-alcohol level was 0.11 per cent.
The judge, who previously denied a motion to dismiss the case, said he based his latest decision not on guilt or innocence, but on whether a conviction would serve any useful purpose in punishing the defendants. He said he also considered their character and personal history.
"I keep coming back to the devastating effect that this event has had on the defendants," the judge said.
"There is no question that some extremely unwise and foolish decisions were made on the evening of Sept. 3 that may have contributed to Alex's death. But Mrs. Montgomery has to live with the fact that her son is gone and her actions on that evening may have been a contributing factor. Mr. Cooper has to live with the reality that his actions may also have contributed."
Alexander, his mother and Cooper were among a group of 14 people from Orillia visiting the fort last Labour Day weekend.
Niagara Parks police said Paula Montgomery told Cooper her son could have a few beers at the campfire.
Montgomery's attorney, Andrew C. LoTempio, said in court last week that the boy had likely snuck more beer to reach that level of intoxication.
"The judge hit the nail on the head," LoTempio said after the charges were dropped.
Montgomery "has already paid the ultimate price. She buried her son. It is so sad. This is really not a victory. There are no winners here."