I think you’d be better off asking that question over on the Apple community. It’ll be an iOS thing, rather than Sonos specific.
I managed to sort this, and thought I would post back in case it helps anyone else…
guided access is a pretty good solution if handing the phone to children etc, but with only a small amount of fiddling, it is easy to bypass.
a much more robust solution is to run Sonos as a single app (kiosk mode). This appears to fix the problem I stated earlier.
Only downside, there is a small cost involved, - but for less than £10 per device, you can have an iPhone or iPad running in kiosk mode.
not having an Apple pc or laptop, I used a product called iMazing for windows, and it seems to work really well. - 30% off too, if you have a student in the family.