I use an RPi4 as the sound generator for the fully weighted 88 key controller my wife plays at home. It runs just one synth - the proprietary physical modelling synth Pianoteq, available for linux, windows, macOS, x86_64 and ARM. It does have a 10" touch screen because the synth has parameters worth controlling (e.g. what piano model). No DAW complexity, but I don't see why you'd ever get a DAW involved with "a toy synth for my 3x 10 year old nephews". It was a little tricky to get the sdcard set up initially, but since I did, it's just power-on-and-play-it.
I think that kids might be interested in more than just a piano, so I wanted to use something like zyn with 16 hand-picked instruments. The nanoKONTROL2 has buttons to play/record/stop, and 8 separate control sections, each with a slider, rotating knob and 3x buttons. I could use them to record into separate tracks and play back solo or together. I just don't know if it's possible to make that usable without a display. That's what I'll be experimenting with. I know that the synth can operate as a VST inside a DAW (eg Ardour) and that many DAWs can run headless (eh hardour) so it may be possible to do this without a mouse, keyboard and screen. The nanoKONTROL2 has LEDs that can be used to indicate status etc.
