How to remove all instances of GetChildren()?

local object = workspace

for i,v in pairs(object:GetChildren()) do
    v:Destroy()
end

Now this deletes everything in the specified object. If you want to delete a specific type of class then you would need to specify that as well using the IsA method that returns whether an instance is a specific class.

An example would be:

local object = workspace

for i,v in pairs(object:GetChildren()) do
    if v:IsA("Hat") then
        v:Destroy()
    end
end

Read more:

IsA

Class Reference

Try setting the variable that you used to nil, I believe once nothing is referencing the list, it will disappear; I've never heard of need to do manual garbage collection in Roblox.

list = Model:GetChildren()
for...end
list = nil

Also, if you're really worried about script/game speed use local variables if you can.

---Edit, after some comment discussion

If you want to remove all items in a list, which is what you get from GetChildren() then use a for loop to look through it

list = Model:GetChildren()
for i=1, #list do
    list[i]:Remove()
end

local items = game:GetService("Workspace").Model

for _, v in pairs(items:GetChildren()) do
v:Destroy()
end

That should work just fine.