Terminating multiple node processes start from NPM run script

I've got the following for spawning multiple node processes when user npm starts:

"scripts": {
    "start": "node calculator.addition/index.js & node calculator.division/index.js & node calculator.multiply/index.js & node calculator.subtract/index.js &"
}

This works fine to start multiple node processes but when I command/ctrl c, the processes are still alive. Any ideas how I can correctly exit these processes?

via anthonyv