Service provider ES5 Ltd, trading as Buggles Child Care Beeliar, was ordered to pay $22,000 and $2000 in costs by the State Administrative Tribunal for contravention of education and care service laws.
A Department of Communities investigation found that in August last year the children left the centre through an outdoor gate.
A mother who was leaving after collecting her child noticed the children and ushered them back inside.
The investigation found educators at the centre had noticed the gate left open more than once during the day and failed to notify the responsible person.
They also failed to ensure the gate was securely locked while the centre failed to notify the Department of Communities Education and Care Regulatory Unit about the incident within the required 24 hours.
"This is the ninth time over the past two years where disciplinary action has been taken against a service where children have left the service unattended, and staff have been unaware of them leaving," Department of Communities Executive Director Phil Payne said in a statement on Friday.
"Childcare service staff must focus on active supervision of children in their care.
"Checks around outdoor areas should include a physical, not just visual, check of gates to ensure they are securely locked."
In other unrelated incidents, a Mandurah centre was ordered to pay $20,000 in penalties and costs after four young children climbed through a fence and were found on the verge of a four-lane highway, in August 2020.
While in November that year a childcare centre at Thornlie left a two-year-old girl unsupervised on a bus for almost two hours.
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