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Kyla (9) and Toby Rae (8) were placed third in their section in the festival’s engineering challenge event after their castle-like structure reached 1.2 metres high.
Kyla said the event was "fun" and Toby had enjoyed also taking part in a family activity.
Their mother Fiona Rae said the paper cup event was simple but gave scope for the two children to decide how best to go about the challenge, and they also learned how to work together on a project.
"I think it was quite well done — all the staff were nice and friendly and encouraging," Mrs Rae said.
Festival associate director Andrea Liberatore said the paper cup building event was one of three "engineering challenge" activities held at the Meridian Centre yesterday, which encouraged participants to "tinker" with various materials until they got things right.
More than 70 people had taken part in yesterday morning’s activities, and there was clear "excitement" as the youngsters tried, failed and improved their efforts.
They were "learning with materials", including straw wells and tower structures partly made out of marshmallows.
This process of continuing trial and error also reflected the scientific method, she said.