Seconding this. Some of the smoothest, least obtrusive apps are on iOS. If you're dead set on Linux I suggest looking at OpenMPT. It's a classic tracker app, v

Seconding this. Some of the smoothest, least obtrusive apps are on iOS. If you're dead set on Linux I suggest looking at OpenMPT. It's a classic tracker app, very mature, and officially supported on Linux through Wine. I was using trackers to make beats at 10 years old, and these days it's much easier to find samples. It's at the right level of complexity for a kid that wants to write sequences and maybe add in some recording, but isn't ready to spend hours and hours on audio engineering.