This was me twenty years ago. So mandatory disclaimer, I was classically trained from the age of like 7 to 20.
I can't stress enough the value of private instruction. Keyboard and voice lessons are relatively inexpensive (not free, of course, but not the biggest luxury) and access to a keyboard or guitar (those especially!) and a teacher who can show proper technique to play and sing is really great. Music is taught primarily in a master/apprentice form and more important than doodling in a DAW is getting someone to show you how to doodle. A relatively inexpensive keyboard and some lessons on playing will go further than any software. Personally I took keyboard and later recording/production lessons and it really gave me the tools to create.
For the tech part, you need three things. A mic to record the voice, a midi keyboard (if you go with lessons you'll need a decent keyboard anyway, and most can be plugged into the computer with USB these days), and a DAW (digital audio workstation). The cheapest form factor now is an iPad with garage band, which is famously user friendly. If you're committed to Linux, bitwig is great but your kid will need to do a lot of self study and practice with YouTube tutorials. Something that can't be beat is a smartphone or tablet with voice memos recording them playing and singing. Audacity can do that.
As an avid Linux user with a lot of experience I will say it sucks for beginner audio work. Get a Mac and use GarageBand, it just works.
If my kid wanted to make music I'd get them a keyboard and lessons, then a voice recorder to make the music. DAWs are essential for pro work but it's a lot to ingest for beginners. Most teachers will be familiar with how to use them these days, ifs standard fare to teach in college.