What is wrong with this script that changes players' characters when they join the game?

Firstly, instead of doing two ifs appended to eachother, do this:

if child:IsA('Accessory')then
       child:Destroy()
elseif child:IsA('BasePart')then
	child.Transparency = 1 
end

How I learned how to use else if Secondly, and the main problem here, is pairs(char:GetChildren()). Pairs is used to parse dictionaries, so it would be looking for a key-value pair like this: {["Roardorak"] = Character, ["Zologo"] = Character} Instead of a key-value pair like this, which is the one you're giving them (where the key is just the index in the array here: {RoardoraksCharacter, ZologosCharacter}. So the solution to your issue would be to simply add an "i" onto the pairs so that it treats the character children like the array that it is, instead of a dictionary.

These two blocks of code should run correctly, but I reccommend trying to code more like the second one in the future.

	game.Players.PlayerAdded:connect(function(player)

player.CharacterAdded:connect(function(char)

   for name, child in ipairs(char:GetChildren()) do
    if child:IsA('Accessory')then 
        child:Destroy()
    end 
    if child:IsA('BasePart')then
        child.Transparency = 1

    end
end

end)
end)

or

game.Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function(char)
   		for name, child in ipairs(char:GetChildren()) do
    			if child:IsA('Accessory') then 
        			child:Destroy()
   			elseif child:IsA('BasePart') then
       				child.Transparency = 1
			end
		end
	end)
end)