The youngsters, from Sunnyside Primary School in Glasgow, had asked if they could take part in Scotland’s International Marine Conference, hosted by government agency Marine Scotland.
But they were told no, because at age 11 the pupils were too young.
They were also forced to hand back tickets to the youth session at the event because they would be the only primary-age attendees present.
This is despite the pupils having received widespread acclaim for their actions to combat plastic pollution in Scotland’s rivers and seas.
They expressed their disappointment on social media as the event got under way in their home city yesterday.
The post said: “So proud of Glasgow hosting the ocean conference.
“As Ocean Defenders we asked to take part; alas we were told no as at 11 we’re too young & had our tickets to the Youth Session taken back as nobody at Primary age will be there. Maybe one day we’ll get to #SeasTheDay. Hope it’s fab.”
But a Scottish Government spokesman said new arrangements have been made so the children can attend the second day of the event.
Speaking last night, he said: “We thank the pupils and teachers at Sunnyside Primary for their commitment to tackling plastic waste and greatly admire their efforts to reduce marine pollution.
“We would be delighted if the pupils were able to join us at the second day of the conference and we are contacting the school tonight to offer our assistance with any travel costs or needs. We’re sorry to have disappointed the pupils at Sunnyside and very much hope to see them tomorrow.”
Local councillors were delighted at the change of heart by organisers.
A spokeswoman for Glasgow City Council said: “It’s fantastic that our Ocean Defenders from Sunnyside Primary will get to go to the conference.
“As active and passionate campaigners, they’ll bring energy and a positive vibe to the conference.”
The children, who set up their own Ocean Defenders team, launched the NaeStrawAtAw campaign, obtaining TV and social media coverage and inspiring local restaurants such as Nando’s and Harvester as well as Glasgow City Council and ferry operator Caledonian MacBrayne to stop using plastic straws.
They have even taken on fast food giant McDonald’s, meeting with bosses in a bid to persuade them to follow suit.
The initiative has been inspirational, prompting other schools to join the movement.