Help with CFrame Loops?

Here's a cleaned up version of what you wrote (better names, spacing, style) (using :children'' is absurd) that does exactly what you did:

wait(3)
local spawns = game.Workspace.Spawns:GetChildren()

for _, dummy in pairs (game.Workspace.Dummys:GetChildren()) do
    wait(0.3)
    local RandomSpawn = spawns[math.random(1, #spawns)]
    if RandomSpawn:FindFirstChild("Used") then
        if not RandomSpawn.Used.Value then
            RandomSpawn.Used.Value = true
            dummy.Torso.CFrame = CFrame.new(RandomSpawn.Position)
                + Vector3.new(0, 1, 0)
        end
    end
    wait()
end

Remark What happens when the spawn you picked randomly has already been picked? You don't move the dummy; the dummy remains where it is, and you move onto the next one.

You instead ought to continue trying until you manage to place it. A repeat until loop is a brief way to accomplish that.

wait(3)
local spawns = game.Workspace.Spawns:GetChildren()
for _, dummy in pairs (game.Workspace.Dummys:GetChildren()) do
    wait(0.3)
    local RandomSpawn -- Declare RandomSpawn's scope.
    repeat
        -- Maybe this one will work?
        RandomSpawn = spawns[math.random(1, #spawns)]
        wait() -- Just in case you run out, instead of crashing,
        -- it will just stop placing.
    until RandomSpawn:FindFirstChild("Used") and not RandomSpawn.UsedValue
    -- RandomSpawn has not yet been used, so use it now
    RandomSpawn.Used.Value = true
    dummy.Torso.CFrame = CFrame.new(RandomSpawn.Position)
        + Vector3.new(0, 1, 0)
end

Alternatively, instead of using Used, you could just remove random objects from your list of spawns, which is much more straightforward:

wait(3)
local spawns = game.Workspace.Spawns:GetChildren()

for _, dummy in pairs (game.Workspace.Dummys:GetChildren()) do
    wait(0.3)
    local RandomSpawn = table.remove(spawns, math.random(1,# spawns))
    -- RandomSpawn has not yet been used -- and is now gone from the list
    -- for future dummies
    dummy.Torso.CFrame = CFrame.new(RandomSpawn.Position)
        + Vector3.new(0, 1, 0)
end

I don't think ":children" is an actual method. I pointed this out in my answer to your forum post and you said it worked, so moving on...

The problem is that if you find a spawn that's used you never try and look for another spawn. You just do nothing. This means that if you toggle one spawn as "Used" and the script randomly picks that spawn, it won't spawn the dummy anywhere.

You can fix that by repeating that by repicking the RandomSpawn until it's not used:

repeat
local RandomSpawn=game.Workspace.Spawns:children''[math.random(1,#game.Workspace.Spawns:children'')]
until not RandomSpawn:FindFirstChild("Used").Value
--rest of the code