This is a test script:
wchildren = game.Workspace:GetChildren()
for a, b in ipairs(wchildren) do
game.Workspace.b:Destroy()
end
However, the game seems to be looking for an object named b instead of one with the name of the variable b.
b is not a valid member of Workspace
Edited: changed code

2 comments
The problem is that b itself is the instance you are looking for. Try print(b.Name) instead and you'll see what I mean. For better coding practices do local wchildren = workspace:GetChildren().
workspace:GetChildren() creates a table of all instances that descend from workspace, and the loop you are using enumerates the items in the table with i, and assigns the item to b.