How would I define the children of a parent?

If you wanted all the children of a parent to have a transparency of 1, this is how you would do it:

local Children = script.Parent:GetChildren() --GetChildren() returns a table containing all the children of the parent
for _,v in pairs(Children) do --This is a "special" for loop that iterates through every element of a table
    if v:IsA("BasePart") then --If the child is a part...
        v.Transparency = 1
    end
end

Hope this helped!

Note: If you still have trouble with GetChildren(), click here: GetChildren()