currently I'm using 28 GB of storage on my pixel 2 (including os install). 32 GB would be a bit cramped, but probably sufficient for the next couple years if she doesn't have tons of games installed.
Well, if you've only got a portion then the only difference between having 10% and 99% is how frequently you have to copy new music on/how long you can go without having to copy new music, at least in my case, where I delete albums off the phone as I play them, and copying music between linux server and android phone via samba/wifi is fast and simple. Streaming - ugh... i use public transport including the tube, I fly (sometimes), I end up in areas without perfect coverage, and I listen to flac so you can't stream that well, and it would take a lot of data, and the choice is limited, and I'd be renting music not owning it forever. Streaming doesn't solve a single problem for me (my daughter loves it, though).
music is an extremely personal thing though. I guess I shouldn't be surprised that people have very different usage patterns.
Can the DAC on your Android actually benefit from the FLACs? Might make sense to re-encode the library into a format better suited to the playback hardware and leave the FLACs at home where the audio hardware benefits from it.
You need really small games, Monument Valley or King of Dragon Path are not especially big games but they're both well above 200MB.
Though you can certainly fit games in way less (Shaun Inman's Horror Vacui and Last Rocket are 1.xMB, Passage is 110K) the data-light games are few and far between.