I think some of the naysayers here are overplaying things just a smidge. As
@Knowlege Bomb noted above, I've seen no reports of a completely bricked phone in a very long time.
The key to successfully running a beta is a way to
stop running the beta if it breaks things you need. This means (A) having an
archived backup (the archived part means that it is stored in a separate location so that it is available to reinstall if something does go wrong; it needs to be archived so that it is not overwritten in the backup folder by a newer backup from iOS 17, which cannot be used to restore to iOS 16) and (B) knowing how to restore from that backup if needed. Easily understandable instructions for both (A) and (B) can be found either in these forums or with a simple Google search. Sites like imore.com are a great resource for this.
I think if most people who haunt these forums regularly step back from the edge of the precipice, they would realize that the bugs they see during the betas are mostly just nuisance bugs: stuttery animations, graphics that aren't laying out properly on the screen, batteries running down more quickly than normal, that sort of thing. In this forum, those things are treated as disastrous and subject to histrionic rants about how Apple can't do anything right. In the real world, these are annoyances and nothing more.
Occasionally, there are apps that don't work or don't work properly, mainly because the apps have not yet been updated to be able to work properly with what is really an unreleased operating system. You can't blame the app developers for this (they are not allowed to update certain aspects of their apps for public consumption until iOS 17 is released to the public in mid-September). You also can't blame Apple for this, as this is part of the process. In any event, there is an entire thread in this forum devoted to surfacing what those non-functioning apps are, and you can/should check there before installing the beta to ensure that an app you need isn't on the list.
So, as long as you are comfortable doing the things in (A) and (B) above (which really are not difficult), a beta is not a scary place. That said, using your granddaughter's SE (provided it is the newer SE that can run iOS 17) is probably a good place to dip your toe in the water.