The 'Strike 4 Climate Action' - inspired by a 15-year-old Swedish school girl's activism - involved children in all capital cities and 20 regional centres.
In Sydney, more than 1000 children - most in school uniform - chanted "climate action now" and "ScoMo sucks" while similar numbers blocked streets outside the Victorian parliament in Melbourne.
A further 1000 students from 100 Queensland schools surrounded the state's parliament in Brisbane, while 20 regional rallies, including at Newcastle, Coffs Harbour and Byron Bay in NSW, saw thousands more take to the streets in protest.
Mount Druitt student Siniva Esera said Australia needs to be the big brother to the low-lying Pacific islands, including for her relatives on Tokelau atolls.
"Our prime minister thinks we should be in school right now, and maybe we should," the Chifley College Senior Campus student told the Sydney protest.
"But how can I just sit by and not do anything to protect the future of this planet and as my family on the islands worry about the rising sea level?"
Forest Lodge Primary school captain Lucie Atkin Bolton said she'd learned in class that leaders need to take responsibility when things go wrong.
"I wish I lived in a country where our adults, especially our politicians, actually cared about my future," the 11-year-old said.
Rose Bay student Michelle Leevig acknowledged issues other than climate change were also important.
"But none of that will matter if the earth ends up drowned, the temperatures rise and there are no humans," she told AAP.
Resources Minister Matt Canavan, who says he's on the side of science and wants Australia to develop all energy sources, including solar and coal, said he'd rather the students learn about mining and science.
"These are the type of things that excite young children and we should be great at as a nation," he told 2GB on Friday.
"The best thing you'll learn about going to a protest is how to join the dole queue."
The series of rallies was inspired by Greta Thunberg, who strikes every Friday outside Sweden's national parliament, demanding the country's leader do something about climate change.