The smartphone has become the modern day version of the pacifier. Pretty much every parent I know hands their phone to their child and it's unsettling how effective it is at pacifying children.

As for this feature request, I think it makes sense... I wouldn't want my kid tapping on my work email or social apps.

I wouldn't want my kid tapping on my smartphone at all. That description of "pacifying" is... both spot-on and deeply sad.

What do you find deeply sad about it?

iPhones have Guided Access which can restrict all usage to a single app:

https://support.apple.com/en-us/HT202612

Modern version of the television, maybe. And why not?

It is a common case for other iOS device(s) though. iPad in a family is usually shared across family members. I do wish apple add this feature in iOS, for iPads at least.

Yep. Two iPads for 3 family members here. It's enough, but you have to remember on which one you left your stuff. If it could seamlessly recognize who picked up the tablet and switch to their stuff, no matter what ipad it was originally on, that would be magic.

I wouldn't even mind having a sync server in the house (a desktop mac, maybe) to help with that.

Yes, I'd really appreciate being able to hand my iPad to my daughter knowing she can only access her apps, and ideally for a set period of time. They seem to have this working reasonably well within macOS users.