To shutdown from the node_helper you need to change some code in the file(will be in MMM-Button/node_helper.js in your file system).
Here is an example from my module to shut down the RPi, you can replace the self.sendSocketNotification(...) in the node_helper with a similar command:

exec('sudo shutdown -h now', null);

Also, at the beginning of the file you need to include the exec function, like here:

const exec = require('child_process').exec;

Or would they only work with the mausberry circuits buttons that attach to the power cord?

´/* global Module */

/* Magic Mirror

• Module: MMM-Button
• MIT Licensed.
  */

Module.register(‘MMM-Button’,{
defaults: {
buttonPIN: 5,
//time in miliseconds before another button click is recognized
clickDelay: 500,
},
// Override socket notification handler.
socketNotificationReceived: function(notification, payload) {
if (notification === “BUTTON_PRESSED”){
this.sendNotification(notification, payload)
}
},
start: function() {
this.sendSocketNotification(‘BUTTON_CONFIG’, this.config);
Log.info('Starting module: ’ + this.name);
}
});´

https://www.pi-supply.com/product/pi-supply-raspberry-pi-power-switch/

I have four buttons conncted to the RPi, and one of them shuts down the system on a long press (>3 seconds).

There is also this module, which you can adapt by adding a shutdown command.