Thanks for all replies. I tested this before the tmux solution, and I must say I'm impressed.

Ctrl-c worked like I was running a single app:

    16:44:33    scss | QUIT
    16:44:33 webpack | QUIT
    16:44:33   nginx | QUIT
    16:44:33      go | QUIT

This was better than expected! I suppose tmux will be preferable if one of the apps is slow to start and must be killed individually.

or Forego! https://github.com/ddollar/forego