An estimated 90 children are being held on the island as part Australia's offshore processing policy.
The asylum seeker, Reza, has told the ABC his crane-top protest is calling for the release of all of Nauru's younger detainees.
"I have just one message from all kids in Nauru. All kids in Nauru need their freedom," he said.
His sign reads: "World: Kids in Nauru need help".
He said he wants to bring attention to the policies of the Australian Government, and the need for it to assist children and families in detention and in refugee resettlement programs on Nauru.
Earlier this week, a bill to remove all children from onshore immigration detention centres passed the Senate with the support of the Greens, Labor and crossbenchers.
The amended Migration and Maritime Powers Amendment Bill (No.1) will now return to the House of Representatives, where it is likely to be rejected by the government majority.
The bill's amendments did not include provisions to remove children from offshore immigration detention centres, such as Nauru.