How can I get all children in a GUI and make those children (frames) invisible?

GetChildren will return an array of Instances that descend one step below the called upon Object. If I create an arbitrary table of elements to represent Instances, you can visually see why this isn’t working:

{1,2,3}.Visible = true

To actually reach the Instance to change a property, you must reference it within the table, or use a pairs loop to traverse to it, or iterate through multiple Instances to apply the same change.

local ScreenGui = script:FindFirstAncestorOfClass("ScreenGui")
local Children = ScreenGui:GetChildren()

for _,Frame in pairs(Children) do
    if (Frame:IsA("Frame")) then
        Frame.Visible = true
    end
end