A 13-year-old boy has been arrested on suspicion of sexually assaulting a four-year-old girl in a children's play centre in Lancaster.
Lancashire Police said the attack took place at The Zone on Dalton Square on 16 February between 12.30 and 1.30pm.
The teenager is currently on police bail pending further enquiries and investigating officers are appealing for information in relation to the incident.
Detective inspector Phil Jones said: "I would like to stress that at this stage, there is no suggestion that any other child has been a victim; we do believe this to have been an isolated incident.
"However, we are seeking any witnesses that may have seen something suspicious or out of the ordinary.
"This is a very sensitive investigation and we are seeking the help of the public. I would ask the parents or relatives of any children who were at The Zone on Saturday between 12.30pm and 2pm to contact the police."
The parents of the toddler are currently being supported by specially trained police officers.
Jones added that The Zone play centre is "fully cooperating" with the investigation.