Kent Fire and Rescue Service said: “Emergency services are working together to search for a young child who has fallen into the River Stour close to Richborough Road and the A256 at Sandwich.
“Crews together with police officers and others are searching the river using the service boat.
“The Coastguard helicopter is supporting from an aerial perspective.”
Kent Police Superintendent Amanda Tillotson said: “Whilst we appreciate the support we have received from members of the public who wish to help in the search, we would strongly advise people to stay away from the water.
“This is for their own safety and to ensure those working within the emergency services can focus solely on locating the missing boy.”
She said police officers, the Coastguard and Kent Fire and Rescue Service remain at the scene as they continue the search.