Why can't I refer to the parent of a child?

So I'm making a GUI backpack script, and it works by creating a button for every part in the "pack" which is a model in Players.LocalPlayer. My problem is, when my update function runs (the function that actually makes the buttons) it keeps making a button for the same item in the pack every time it runs. I am trying to make a second pack so that I can move the items to that pack once a button is created for it, but I'm having trouble moving them from pack to pack2. The only way I can think of referring to the item that I want to move is child, since this is all happening in a "for index, child in pairs(pack_tab) do" When I refer to the child it says " bad argument #3 to 'Parent' (Object expected, got string)" So I guess to simplify my question, why can't i refer to the parent of a child, and how can I get around this problem? here is the function I'm talking about below. Let me know if you have any questions because I know this is kind of complicated and confusing. Thanks!

function Update()
    local player = game.Players.LocalPlayer
    local char = player.Character
    local pack = player:findFirstChild("Pack")
    local pack2 = player:findFirstChild("Pack2")
    local pack_tab = pack:GetChildren()
    local Ipack_tab = IFRAME:GetChildren()

    if CURRENT == nil then
        local x = 0
        local y = 0


        for index, child in pairs(pack_tab) do
            local CHOICE_GUI = MFRAME.TextButton:clone()
CHOICE_GUI.Size = UDim2.new(0, 100, 0, 25)
            local new_gui = CHOICE_GUI 
            new_gui.Position = UDim2.new(0, x, 0, y)
            new_gui.Text = child.Name
            new_gui.Name = child.Name
            new_gui.MouseButton1Down:connect(function() onGuiClick(child) end)
            new_gui.Parent = IFRAME
            new_gui.Visible = true

            y = y + 25

            if y > 350 then
                y = 20
                x = x + 100

            end
            local CHILD = pack:FindFirstChild(child)
            CHILD.Parent = "Pack2"
        end

        --cleanGui()
    end
    end