I was wondering if there is a workflow that she can use on her laptop (running Pop!_OS which is based on Ubuntu) to easily create, say, remix of songs. I have heard of Jack and have occasionally used Audacity but I neither have any background in creating music nor any specific equipment that is needed (I'm not opposed to buying if this turns out to be a long-term interest for her). Also, it doesn't have to be offline - purely web-based or combination of online/offline is fine.
My main goal is for her to be able to create quickly and easily while keeping the complexity low so as to not scare her away.
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.
I'd like to mention SonicPi though. It's a platform for creating music through code, and is used all over the world to teach kids both programming and music production at the same time. It's really cool.
It should be installable via apt, so it's easy to try it out, anyways!
Bitwig is one of the only major audio workstations taking Linux seriously. They're basically building a supercharged version of Ableton Live, and it's got great multiplatform support. If latest PopOS! has PipeWire enabled, then all you need to install is your pipewire-jack package for the audio server, and your bitwig-studio package for Bitwig. Once you have Bitwig installed, you can boot that up and activate it with whichever version you get. It's a seriously powerful tool, and pairs nicely with touchscreen devices. Works great with MIDI enabled hardware, too!
Keeping complexity down is going to be the main issue (isn’t it always). Music software can get very complex unfortunately.
Is your daughter set on doing this “in the box” (on computer)? My first thought is that physical equipment might have easier (or at least more satisfying for a child) learning curves.
Hopefully someone can hop in with concrete equipment for mixing or Linux software advice. Mac guy myself.
Music theory is optional.