The state police office in Bavaria, southern Germany, said in a statement that the boy and his father had discovered the pipes in a sandpit and under a bush at the playground in the town of Fürstenfeldbruck.
The father reported the find to the police, noting that he had heard a loud bang at the site the night before and saw three young men climbing into a car and driving away. He wrote down their number plate.
Explosives experts determined the pipes contained live bombs.
Using the number plate details, police tracked down the three men, aged 19 to 21, the statement said.
"All three confessed to having built the bombs themselves using power from leftover New Year's Eve firecrackers," it said.
"They were charged with violating explosives laws."