game.Shutdown is locked. and if you simply kick each player even OnPlayerAdded the server won't shutdown. It will wait for players.
Let´s make it a GUI button for this example. And the path would be game.StarterGui.ShutdownGUI.ShutdownButton.ClickScript.
So, the ClickScript has this in it:
script.Parent.MouseButton1Down:Connect(function(Shutdown) game.workspace:ClearAllChildren() game.Players:ClearAllChildren()
ROBLOX Studio should automatically add any ends that are needed.