"As long as
airplay is supported by Sonos, Sonos will work fine long term too."
Sonos has no incentive to kill airplay. Instead they are
focused on speakers and only speakers... so they support Airplay just as fully, along with
HomeKit,
Apple Music, and
all of the other music sources too (unlike HPs). And in this concept of leaning on Airplay for the future, Daughter can lean on the HP "smarts" in the same devices she would lean on for Airplay: the AppleTV she already has, the iPhone and/or iPad and/or Mac she presumably already has too.
For home theater purposes (as Santa intends) 4 or 5 HP Minis would be only up to 2 for the TV... and thus only stereo at best. ARC would be front left, right and center channel in one purchase, with the flexibility to add other speakers for true surround and a SUB for deeper bass if daughter wants either. And even though OP dismisses center as important, in home theater, center is mostly for crisp, clear dialogue vs. faking it through a faux center generated by 2 stereo speakers. Daughter will hear what characters are saying on her TV much more clearly if she has a true center. One might argue that center is actually MORE important than left & right for that purpose.
If Santa wanted to give daughter easy access to good-sounding music all over the house, I could get more behind the HP suggestion (though I'd still favor Sonos Move over HPs myself). But the focus is on home theater and limiting daughter to stereo at best is quite the cap on her TV audio potential.
And yes, the best option- Receiver + some "Dumb" speakers- is more complicated to set up
ONCE. But once it is in place, it is about as easy to use. I click one button to turn my setup like that on and one button to turn it off. Volume up & down controls the volume. Etc. The big advantage to going to that trouble one time is that it can easily evolve into a full home theater setup (including true not faux ATMOS if desired) AND "dumb" speakers can never become obsolete by either company deciding to dump them by changing their "smarts" on which smart speakers depend. Sonos and HP are like iMac: when any one part is made obsolete or fails, the whole thing has to be junked. "Dumb" speakers are the ones that can confidently last many decades with reasonable care.
All that offered though- if "Santa" is really looking for rationalizations to give daughter HPs- and only HPs- and what they wrote in post #1 was just a smoke screen, if I was "Santa" I'd be giving her full-sized HPs for Home Theater purposes vs. Minis. The bigger ones do sound better and TV audio should sound as good as anyone can afford since that tends to get used every day... for years and years.