Just wondering, are you using a local script or a server script? Because if you are using a localscript, the new material will only show to you.
Also, its better to check if the part name is lag before changing the material
script.Parent.MouseButton1Click:connect(function()
for i, part in pairs(game:GetService("Workspace"):WaitForChild("rmv"):GetChildren())
if part.Name == "lag" and part:IsA("Part") then
part.Material = Enum.Material.Neon
end
end
end)
for i,v in pairs (rmv:GetChildren()) do --get all children from model
if v:IsA("BasePart") and v.Name == "lag" then --check childrens name and classname
v.Material = "Neon"
end
end