An investigation is underway after a young child went missing from a childcare centre in Sydney's south for several hours on Tuesday, in an incident the acting education minister has called "unacceptable".

The toddler's mother arrived at First Steps Learning Academy in Bangor yesterday afternoon to discover her child was not there.

Police sources told the ABC an educator handed the wrong child to a grandparent, who took the infant home before returning several hours later.

The childcare centre on Wednesday told families the "serious incident" had taken place at 3:40pm after a "carer, for a child in the nursery room, at our service, mistakenly picked up the wrong child from the centre".

"Emergency processes were followed to track down the whereabouts of the child," the email from the Ba

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