Is there a way to Passcode the Cellular Data button to OFF on an iPhone SE

Why? It would seem like a better idea to control that via your carrier. Most will allow you to suspend data service to a particular line on your account. Some will allow you to set 'limits' that will automatically suspend cellular data use when it reaches a set point on a particular line to avoid overages. Turning off cellular data and locking it down means a number of features will not work reliably, or in some cases, at all.

Go into the settings/general/ restrictions. You will have to create a password. Once in enabled you will have the option to choose "don't allow changes".