generator.lua

local function iterateChildren(instance)
  local children = instance:GetChildren()
  local index = 0

  print("creating a generator")
  
  return function()
    print("invoking the generator function")
    
    index += 1
    return children[index]
  end
end

local folder = Instance.new("Folder", workspace)
Instance.new("Folder", folder).Name = "1"
Instance.new("Folder", folder).Name = "2"
Instance.new("Folder", folder).Name = "3"

for child in iterateChildren(workspace) do
  print(child.Name)
end
--> creating a generator
--> invoking the generator function
--> 1
--> invoking the generator function
--> 2
--> invoking the generator function
--> 3
--> invoking the generator function
-- here it breaks the loop since the generator returned nil

-- Note that this generator does not have to go by order too
-- so it can be 3, 1, 2 or 2, 3, 1 ...

-- You can also write it as this
local generator = iterateChildren(folder)

for child in generator do
  print(child.Name)
end