A 20-year-old nursery worker has been arrested on suspicion of child abuse, police have said.
The man, from Nechells, Birmingham, was taken into custody in the early hours of Wednesday morning, West Midlands Police said.
Assistant Chief Constable Anil Patani said: “The investigation is in its very early stages, however we believe that we have identified one victim who we believe attends the nursery.
“The family of this child have been told and police are supporting them at this very difficult time.
“As the investigation continues, we will update families with children at the nursery at the earliest possible opportunity.”
Police said the man was employed as an assistant at the Little Stars Nursery in Nechells.
A force spokeswoman said the nursery was closed “as a precaution”, adding: “West Midlands Police and Birmingham City Council are in the process of contacting parents and guardians of children registered at the nursery.”
Senior police officers and council officials will be holding a private meeting for parents, police said.
A telephone line offering advice and support to parents and guardians has also been set up. The number is 0121 675 4806.