Not at all. Chrome OS would be a better fit there.
Unless you're teaching your kids & grandparents how to sysadmin linux and don't think they want to watch any videos online (browser support for accelerated video decoding remains a giant disaster on linux), then sure? And also that they want a desktop instead of a laptop.
Chrome doesn't support hardware acceleration video decode on Linux at all. There's some unofficial patches for VaAPI support if you want to carry those around, but the Pi 4 also changed decoder support and it's generally all just a messy disaster.
Low-res video can be software decoded fine, but once you start pushing 1080p or 4k it's not going to go well at all.
Of course a "ASUS CHROMEBOX 3-N018U" runs over four times the price, so you could just buy three pi and toss them as the flash card fails and still save money.
There's no shortage of chromebooks in the $250-300 range which then also include a display, narrowing the price difference drastically.
Of course there's also used options, which is also a good option for ipads and such that would still be a much better option than this for a non-techy user.
The Rpi 400 looks fun, but it's a tech toy for hobbiest electronics usage or general hackery. Not a generally good computing appliance for the masses.
Unless you're teaching your kids & grandparents how to sysadmin linux and don't think they want to watch any videos online (browser support for accelerated video decoding remains a giant disaster on linux), then sure? And also that they want a desktop instead of a laptop.