GUI script can't find children to tell them to become Visible?

A continuation of my problem from here.

I need my MakeVisible script from before (see link above) to make my GUI visible. It does make its parent GUI Visible, however, the Children of the GUI I also need to be visible aren't becoming so.

Here are the Children I need to make Visible (Close, Next, Previous, and Cover): https://i.imgur.com/ksLXqZh.png

Below is my MakeVisible script that should be making these Visible. This is being triggered via a RemoteEvent. If you want to see those scripts, see the link to my other problem above.

local ReplicatedStorage = game:GetService("ReplicatedStorage")

local remoteEvent = ReplicatedStorage:WaitForChild("DinosaurBook")

local GUI = script.Parent

local cover = GUI.Cover

local closebutton = GUI.Close

local nextbutton = GUI.Next

local previousbutton = GUI.Previous

local function onNotifyPlayer()
    GUI.Visible == true
    cover.Visible == true
    closebutton.Visible == true
    nextbutton.Visible == true
    previousbutton.Visible == true
end)

remoteEvent.OnClientEvent:Connect(onNotifyPlayer)

I also tried a WaitForChild, however this didn't work.

Note that my PlayerGui is Enabled: https://i.imgur.com/Uimz9vV.png

And the Frame is Visible after the ClickDetector registers the click: https://i.imgur.com/dfa3xJ4.png

But the Close button (for example- Next, Previous, and Cover also do not become Visible) is not Visible: https://i.imgur.com/YzVDzWE.png

I imagine there's something wrong in my script that it can't find the GUI's Children, but it can find the GUI itself?