The paddling pool by the West Pier in Brighton has been closed after the water became contaminated.
Routine tests have found raised levels of bacteria in the water, which could have come from birds, dogs or humans.
Brighton and Hove City Council said that the pool was expected to remain closed until Friday (21 June).
The water has been drained to enable staff to clean the pool floor and filtering systems.
The council said that other paddling pools were available at Hove Lagoon and Saunders Park in Brighton.
It added that the water play facility with fountains was also open near the Yellowave beach sports centre in Madeira Drive.
Council executive director Geoff Raw said: “We’re really sorry but this is always a risk with an outdoor pool.
“It’s extremely bad luck that the incident coincides with what could be some of the warmest weather of the year so far.
“We’ll work to get it open as soon as possible and in the meantime we’re reminding families there are other places where children can have a splash-about.”
