The new City of Perth library has inexplicably cancelled its Harry Potter-themed event for children because adult fans complained about not being allowed to go.

Perth leapt to attention when the library announced the event, lured by the activities such as meeting and learning about owls from the WA Bird of Prey Centre and attending a 'potions class' with scientists from the ChemCentre.

Over 18 Perth fans were not impressed.

The library tried to explain to the 11,000 people who expressed interest that the event was just for children.

But thousands retorted that the "real fans" who grew up with Harry Potter were now in their twenties and early thirties.