Investigators with the Ventura County Sheriff's Department said two children were playing in a tunnel at Santa Susana Park on Thursday when a man followed them in it.
Authorities said the man then committed a lewd act involving one of the children. Authorities did not release details of the incident.
Neither child was harmed.
The suspect is described as white, 40 to 45-years-old with a beard.
Anyone with information is asked to call Crime Stoppers at (800) 222-8477.